To clarify what I meant regarding Britain: when I said that Constantine III. wasn't a factor, I meant that he wasn't one before the Crossing. Thereafter he became a major factor, but before that it was Radagasius (as you said) who had force Stilicho back into Italy and away from the future area of conflict. And while Britain had experienced trouble in the 360s, by the 400s it seems to have been calm enough for Constantine to consider moving his army into Gaul.
This matter is complicated by the fact we are not sure when the crossing (of the Rhine) took place and when the usurpation did. It's likely that the crossing preceded the usurpation (thus providing an additional reason, among the many others, for it). But yes, in the end it was a combination of too many issues blowing up on Stilicho's face at the same time, bad timing, the surprise effect of the tribes crossing the Rhine (which no one had accounted for), and finally Stilicho not being given the time to deal with any of those threats. Compared to this, the situation in the East was marginally better (at least in the field of leadership I would say), though there is still room for similar blunders.
Hence why the importance of the pod Heraclius is a very competent emperor but he will die in early 640s it really depends a lot if Constans II is born if he is then the empire should be fine as he was also a competent emperor.
I can't say for sure if Constans was 100% up for the task (his campaign in Italy was honestly a little underwhelming), but he was definitely no Honorius.
if you are referring to Jonh of nikou John of Antioch there is a really good I recommend Christian reaction towards Islam in the 7th and 8th centuries by Doç. Dr. Đsmail while its a possibility something would have to go terrible wrong ( like enacting bloody persecution ) to alienate from the empire
I actually was talking about John of Ephesus. And by Arabs I meant the VI century Christian Arabs. Basically John viewed the leader of the neighbouring Arabs (a fellow anti-Chalcedonian) more favourably than the emperor of the Romans Justin II. If the invaders are actual Christians (the branch of Christianity popular in the East) instead of Muslims, I would have no problem believing in their ability to gain friends and supporters among the locals against the Romans (despite the locals also being technically Romans).