The immediate circumstances of Simon's overthrow in August 1265 were eminently avoidable and seem to suggest a mixture of bad luck and carelessness - especially by his subordinates (led by his feckless and arrogant sons) who let him down. Firstly he heads off with the captive Henry III and Edward to Hereford, some way W of the R Severn (then crossable at only a few points) to deal with trouble from the royals' allies the Marcher barons,who were defeated with H and E at the battle of Lewes in 1264 and signed up to the new order but only as long as they were not interned - and were supposed to go into exile on their lands in Ireland for a year but failed to go. Simon cannot make them, and in summer 1265 the King's unpopular and land - and office-grabbing half-brothers the Lusignans, who S has exiled, illegally return to SW Wales. They start a revolt, and are backed by Simon's young and powerful ally Earl Gilbert de Clare of Gloucester (the 'Red Earl' from his hair and temper), aged 22, who had quarrelled with S's sons. Gilbert was a reformist ally in 1264, but now protests at S's and his sons' abuses of power, eg keeping royal castles and a mercenary army (necessary to hold Henry and the more dynamic Edward in check). Simon tries to negotiate with Gilbert, unaware that local Marcher barons have joined the plot; then his hostage Edward is allowed to exercise on his horse in races against his guards on the meadows over the R Wye from Hereford castle, a visiting De Clare negotiator (Gilbert's brother) slips him a fresh horse once his guards' horses are tired out, and he breaks for freedom and gallops off to a rebel castle (Wigmore). The guards cannot catch him, and he and Gilbert promise to keep Simon's reforms but kick out his greedy family from monopolizing power.
This is a major slip by usually alert and wily Simon; then S stays on W of the Severn to negotiate a treaty with his ally Llywelyn of Gwynedd (the new overlord of the other Welsh kingdoms and keen to force S and Henry to recognise this). L will take the Marchers in the rear. But while the treaty is being finalised, E's rebellion spreads unchecked; he and his men take and destroy the Severn bridges down to Gloucester, cutting Simon off from central England (his HQ is at Kenilworth Castle nr Birmingham). Simon heads off down the Bristol Channel to get ships to cross to Bristol; he reaches Newport on the R Usk in a hurry, and fails to set guards (too tired?) so Edward arrives by sea and burns his ships under the castle windows. Simon has to struggle back NE across country, slowly, and find a ford over the Severn upriver nr Worcester and cross; meanwhile his son Simon jr is supposed to bring an army from London to help but has gone off on a tangent to sack pro-royalist Winchester so he arrives late. He was supposed to be at Bristol to meet Simon as the latter landed by sea; instead he reaches Kenilworth late, and his men are allowed to camp in the town and enjoy themselves not go into the cramped but safe castle. Edward arrives at night and smashes their army, driving the survivors into the castle; he steals their heraldic banners and sets off to intercept Simon who's crossed the Severn. S arrives at Evesham, sees his son's banners ahead and thinks it's help, and relaxes; Edward closes in, then reveals his own banners at the last moment. The Montfortians panic; the Marchers , aiding Edward, cut off their retreat so they have to try to break through Edward's line; he has the stronger army and destroys them in a bloodbath, rare for a medieval battle where knights are normally taken for ransom but on this occasion many are killed. Simon is hacked to pieces by the Marchers, seen as shocking by contemporaries, along with his eldest son Henry - and in revenge six years later his exiled sons Simon jr and Guy will murder Edward's cousin Henry 'of Almain' in a church in Italy to get back at Edward.
Had Simon hurried the Welsh treaty up, had better troops to guard the Severn crossings, or set guards at Newport he would have escaped Edward's trap, and had Simon jr kept to schedule and set guards at Kenilworth he would have escaped attack and joined his father sooner; then they would have had the stronger army and had a good chance of winning. If Edward is killed the revolt collapses; if he escapes he is left stranded in France with his mother Eleanor, the N Italy (Savoyard) sister of King Louis IX's wife who has been trying to get Louis to invade. The Marchers are defeated in any case and S is safe; England is ruled by a committee of barons in the ageing Henry's name, at least until H dies in 1272. Then Edward is disinherited, as S had occasionally threatened earlier, and either his weaker and less militarily capable brother Edmund (aged 27 in 1272; Edward was 33) is made puppet-King or more likely Simon installs their cousin Henry 'of Almain', son of H III's brother Richard (who d earlier 1272) and a more generous and reformist person than Edward. Henry of A had expressed support for Simon for most of the 1258-64 crises, but eventually backed his kin in 1264 - but afterwards S trusted him to help negotiate with the Pope, who backed Henry and sent envoys ordering S to restore him to power or face invasion.
If Henry of A is made King, the Pope refuses to recognise him (most Popes declared any restraints on royal power illegal, eg Innocent III backing John not Magna Carta in 1215) and calls for an invasion to restore Edward. But the probable leaders of such an invasion as of 1265 are unavailable by 1272 - Louis IX goes off on Crusade in 1270 and dies in Tunisia and from 1265-6 his brother Charles of Anjou is busy in S Italy, conquering Naples and Sicily as a new kingdom for himself with Papal backing. Charles was the expected leader of an attack on Simon's England in 1265 in OTL, as a 'Crusade' backed by the Pope, but due to the Naples crisis - more of a priority for the Pope - this was called off. After this he and his troops are unavailable, and unless Louis (hostile to S but more interested in a crusade) invades in 1266-8 to help Edward he's unavailable too. So E will have to rely on revolt in England - always possible given inter-baron feuds and the unpopularity of Simon's aggressive sons, who in OTL Simon blamed for the summer 1265 revolt (mainly by alienating Earl Gilbert of Gloucester). A governing committee of barons taking the royal powers plus probably annual assemblies of the elite from small landholders to major barons (ie Parliament), as arranged in S's constitution of 1264-5, was unprecedented in England under an adult ruler, and quite apart from foreign meddling led by the Papacy/ France would probably fall victim in a few years to internal strife. At the best it would not survive once S was dead, given how few others were idealistically committed to it; and the problem of constraining a king by a committee of barons was always what the king would do when he recovered his freedom of action. (Probably arrange a bloody purge as Edward II did in 1322 and Richard II did in 1397-9, leading to revolt later.)
If the earl of Gloucester does not join the 1265 rebels they have less troops and force, and probably lose; and though Gilbert and Simon's greedy, aggressive sons hated each other and indulgent S failed to rein his sons in the crisis could be avoided if (a) Simon had more moderate and co-operative sons or (b) Gilbert's more restrained father Earl Richard, a friend and reformist ally of Simon in the 1250s, had not died suddenly aged 39 after a banquet with the Queen's uncle in 1262. (This was rumoured to be poison and the cook was hanged, but given poor hygiene food-poisoning is more likely.) If either or both happen, Edward - unreliable, devious, willing to violate oaths, and furious at S for constraining royal power - would not accept the continuing rule by baronial committee that S insisted on continuing after H's death. At some point he would have to be released from custody and would end up in rebellion, and once H is dead he can claim to be the legitimate king even if S has his cousin elected as King instead as more trustable. If he is killed , his cousin Henry of Almain might survive as king - abandoning the rule by committee with elite support outside S's circle, but keeping to the constraints of seeking advice and wise rule which S insisted on. We might even get Edward, back from Crusade in 1274 as in OTL and a Papal champion, invading England with French troops loaned by Philip III (r 1270-85) to overthrow his cousin - but probably Philip would insist that he paid for the help by handing over Aquitaine so he loses his French lands.
As for the Jews - Edward was even more anti-Semitic and implacable than Simon and expelled them all in 1290, partly as a cynical way to avoid paying his debts to them, and other English regimes unable to pay their debts had a habit of cancelling them and/or expelling the bankers instead (eg Edward III in the 1340s with Italian banks). A pogrom was an easy way of gaining a good reputation with the Church as a devout 'Crusader', though a 'King Edmund' (Edward;s brother) or 'King Henry IV' (of Almain) from 1272 would be less ferocious than E and it might be delayed until another major financial crisis (war with France?)
