Were the Istari Unable or Unwilling...

[Raynor]

Almost certainly not Manwe, I don't think he would have had the authority to recall one of his own order from what was a very real "death". Did the Head Honcho Eru step in?

Indeed, it wasn't Manwe (emphasis added):

Originally Posted by Letter #156

He was sent by a mere prudent plan of the angelic Valar or governors; but Authority had taken up this plan and enlarged it, at the moment of its failure. 'Naked I was sent back – for a brief time, until my task is done'. Sent back by whom, and whence? Not by the 'gods' whose business is only with this embodied world and its time; for he passed 'out of thought and time'.

was he made stronger, or was it just the case that the prohibition was relaxed (even a little) for him?

He was indeed made stronger (and wiser) - and the rules received an, hm, emendation?

Originally Posted by Letter #156

Of course he remains similar in personality and idiosyncrasy, but both his wisdom and power are much greater. When he speaks he commands attention; the old Gandalf could not have dealt so with Th eoden, nor with Saruman. He is still under the obligation of concealing his power and of teaching rather than forcing or dominating wills, but where the physical powers of the Enemy are too great for the good will of the opposers to be effective he can act in emergency as an 'angel' – no more violently than the release of St Peter from prison.