I've covered the issues regarding the Tripartite Indenture in one of the chapters of my Alternative History of Britain: The Hundred Years War (Pen and Sword, 2013). Briefly, I don't think a formal division of the state of England between three equally ranked powers - Owain Glyndwr to have all of England West of the Severn river and the Trent plus all of Wales, the Earl of Northumberland (as head of the Percy clan) to have all the North to the Trent-Humber, and Edmund Mortimer to have the rest - is plausible. Glyndwr had a large but not that well-armed Welsh rebel army with not enough manpower let alone proto-cannons to take over the remaining pro-Henry IV Welsh and English castles and towns in S and E Wales, let alone the Severn valley where there was no Welsh settlement or signs of anti-Henry IV gentry and townsmen in 1405, and a large bloc of the local gentry and their tenants were still fighting for Henry and his son Prince Henry (later H V) as Prince of Wales in a viable army in 1405 - the temporary advance of Glyndwr as far as the outskirts of Worcester on one march was due to a large and coherent, well-armed French contingent sent by the then sane Charles VI and his anti-English brother the Duke of Orleans, and this army lacked enough numbers or cavalry or archers to tackle Prince Henry had he been in the district at the time - their success was a one-off and any attempt to conquer territory in the lowlands of SE Wales or the Severn would have needed a far larger and permanent French contingent plus major Eng defections (both v unlikely) and an ability to defat the Prince and his skilled local gentry lieutenants.
In the North, similarly, there is just too much local armed and gentry-led opposition to the Percies for any conquest by them and their local Neville family foes (lords of most of Cumbria and with a large force of expert fighter Border tenants) would have opposed them, as would all the Percies' other enemies. If the Percies had brought in Scots help (very unlikely as king Robert III, d 1406, was elderly, infirm, and a chronic depressive, his brother and regent the Duke of Albany was far too cautious to invade England and his son was Henry IV's hostage, and his only adult son Prince David had died in suspicious circumstances as Albany's prisoner in 1402) this would have united local hatred and led to strong resistance. In any case the Percies had lost much of their manpower at the battle of Shrewsbury vs Henry IV in their last rebellion in 1403 and had had much of their lands confiscated; the earl of Northumberland was now a lot weaker and was little better than a Scots hanger-on relying on them and a few loyal lords for help. The Percies would have been in a far stronger position if they had not rebelled in 1403, with N's son 'Hotspur' and brother the earl of Worcester still alive as both were good generals, but H IV had already defeated them once and N had had to sue for mercy to get some of his lands back.
So GlynDwr and Percy were weaker than they looked in 1405-6 and France was an unreliable ally, likely to be distracted soon by another bout of civil war; nor was there any legal precedent or elite support for breaking up the English state. Mortimer, aged only 13 in 1405 so only an inexperienced puppet-king, might have been a nominal King and front-man for Percy and (at a distance) GD and hjs allies, helped by a French elite knightly force for a while, if he had escaped from Henry IV's custody in spring 1405 to join the rebels and they had then won a major battle and killed H IV - in real life he did escape with his brother from internment in Windsor Castle, a few months before the Indenture, as his governess got hold of the keys to the gates and arranged for horses to be ready. They bolted for S Wales, but were captured by Henry's men in a wood near Cheltenham and dragged back to Windsor; had they reached safety there was a lot of popular feeling, incl among legalistic Churchmen, that Henry had cheated Mortimer of the throne in 1399 when Richard II was deposed but this did not translate to active military revolt by large numbers of elite figures.
Had Henry IV, whose health collapsed with a mysterious sickness a few weeks after an anti-tax revolt broke out in Yorks led by the pro-Mortimer Archbishop Scrope (who provided Church support and called H a liar and usurper - he was then executed), been incapacitated durign not after the revolt he could have lost his throne as his army collapsed. In real life he and his allies were able to defeat the rebels first - though his executing the archbishop then led to claims that his illness was God's punishment and thereafter (he was only 38 or 39 at the time) he was sporadically ill and incapacitated and his power was duly undermined, though his strong sons and cousins helped his regime to survive. But the pro-Mortimer rebels in the NE would still have had to defeat Prince Henry and his army, though this chance - H IV falls ill earlier and his army panics - is probably the best chance of a Mortimer victory, Especially if Northumberland has not revolted and been defeated in 1403 so he has more men to help Scrope.
But if Prince Henry is still alive and in command of a large army he is stronger and better supported than either GD or the hated and only temporarily present French army - Mortimer might have to come to terms with Pr Henry and so probably 'cut out' GD, except as ruler of those parts of Wales which he already occupies. Handing over any more of Wales to him is unlikely - the local Anglo-Norman gentry and peers had large castles and private armies and would fight on allied to Pr Henry so Mortimer would logically have had to give in and insist on GD accepting a compromise. Pr Henry would lose his own occupied Welsh lands - but he could be given the Duchy of Lancaster, as confiscated from his deposed father H IV. And as Mortimer never has a son, logically Pr Henry has to be made his heir to win him over and he could still revolt later.
