On the question of 'Henry IX', ie the elder son of James VI and I who died aged 18 in 1612 , apparently of typhoid after swimming in the River Thames:
As I have explored in a chapter pf my current Sealion ebook 'King Henry IX', I think Henry surviving to succeed James in 1625 would have profound results on UK history , but not necessarily avoid the Civil War. As mentioned, he was a strong Calvinist so he would not back the supposedly 'Popish' (as feared by opponents) ceremonialism of Archbishop Laud in the Anglican Church after 1633 and so arouse Scots Calvinist opposition as well as that of English 'Puritans'.He would probably not even make the theologically suspect Laud archbishop of Canterbury or promote any non-Calvinist bishops unless they were congenial to him as disciplinarians, and would certainly not risk increasing Anglican-style bishops' powers, or Prayer Book theology in either England or Scotland. He had been brought up in Scotland until he was 9 and then had had Scots close to him in his household in England,, so he understood their fears (his brother Charles had left Scotland when he was 2). At most, a disciplinarian like him would have been irritated by autonomous Calvinist groups in the English and Scots Churches as disrupters and cracked down on this - as James did after he came to England. So no Scots rebellion in 1637 - but given his anti-Catholic attitude and fear of expansionist Counter-Reformationary Catholicism on the Continent he could well have extended Protestant colonization in Ulster or elsewhere in Ireland. So an earlier Irish Catholic revolt than the real one in 1641, then a stronger Royal reaction than was possible in real life from crisis-hit Charles I? A Cromwell-style 'plantation' by 'godly' English Protestants after Henry defeated the rebels?
Henry was in real life interested in European affairs and alert to the 'Catholic expansionist' threat masterminded by the Austrian and Spanish Habsburgs, and backed the abortive plans of French King Henri IV for a German Protestant alliance backed by France to stop this. Henri was assassinated and then Pr Henry died, but in 1618-19 Henry's favourite sister Elizabeth and her German Prot husband Frederick of the Palatinate tried to take the kingdom of Bohemia off the Catholic Habsburgs and were driven out of B and then the Palatinate. This sparked the Thirty Years' War off; would-be mediator James I stayed out of it but eventually lent Elizabeth and Frederick troops to try to get the Palatinate back after mediation with Spain failed. In real life Charles stayed out of the war once he was king, and ended up fighting his wife's brother Louis XIII of France over French Catholic suppression of the autonomous towns given to the French Protestants (ie Huguenots) instead in 1627-8. (This is the setting of Dumas' 'Three Musketeers' stories and the modern films of it, with the English under Charles I's minister Buckingham trying to rescue the Huguenots besieged at La Rochelle.)
Would the more determined and martial Henry, who unlike Charles was a confidant horseman and rode in tournaments, have led his army to intervene in Europe in 1625-6 once he was King and aided the German Protestants? Could they have stopped the Catholic advances in the late 1620s with Henry's help, and so not needed the intervention of King Gustavus Adolphus of Sweden to rescue them? Or could Henry and GA combined have defeated the Habsburgs and forced the Emperor to cede large amounts of land to the Prot princes, redressing the balance of power in Central Europe? Or would Henry have rescued the Huguenots and stabilised their power as autonomous communities with guaranteed legal rights in France, humiliating (and causing the fall of?) the centralising chief minister Cardinal Richelieu? Either English success in Germany or in France would however rely on Henry having success in reforming, training, and supplying the army and navy far better than Charles did in real life, which would not be impossible given his administrative zeal and the financial support of Parliament if this liked his fervent anti-Catholic rhetoric. Unlike Charles, Henry was in tune with majority elite opinion in England about the 'Catholic threat' so he would not have been dissolving and ruling without Parliament in 1629 - but wars have to be paid for and his ambitions would be very costly. Like Charles, he does not seem to have been inclined to compromise.
In that case , perhaps he would annoy MPs by demanding too much money too often and by centralising administrative reform, rather than by religion etc as Charles did. So England would end up with a crisis of governance between MPs and a strong and demanding King after all in the 1640s - but in a far different context . Notably, some of the real-life Parliamentary generals in the 1640s (eg the Earl of Essex) were Henry's friends as young men, or had similar religious views - so in a crisis they would probably back him. And would zealous Puritan and failed Fenland farmer Oliver Cromwell be busy as a 'planter' in Ireland not in England in the 1640s?