Was it that popular with pagans, though? I don't remember a lot of pagans rallying behind his reforms, though maybe I just haven't read enough about that part.
It's hard to say, Julian started implementing his reform in the period of time between Constantinople and Antioch, not enough time to gauge a proper reaction. Realistically he would have encountered some sort opposition, since he was expecting the new pagan clergy to lead by example (with rotten apples forced to leave). On the other hand the promise of renewed imperial support for traditional cults (in the form of funds) was sure bound to attract renewed interest and change of sides. This is mostly referring to the aristocracy involved in maintaining the cults. The average citizen of the empire would have probably just cared about occasions for festivals and the new clergy providing help/shelter (one of the objective of these reforms) in the same way as the Christians.
I believe the most likely reaction (among the pagan aristocracy) was that the old cults did not really need such reforms (but were not really opposed to them either). It was not really something new either, as I believe Maximinus Daia tried something similar at the start of the century. The real point of contention (but not the only one) was mostly the excess of blood sacrifice.
To be fair one might get that impression were he led the worst offensive against Persia that makes Anthony Parthian campaign look like a genius
As you said, that is just an impression. There are a lot of articles out there examining the various aspect of Julian's campaign in Persia. To summarize some of them:
Julian successfully achieved the element of speed and surprise;
The burning of the fleet was no "calamitous mistake", since the fleet could not sail upward the Euphrates. The task had been accomplished and so the fleet could be burned;
We don't really know how Julian planned to conclude his campaign, with the siege of Ctesiphon or one final big battle. If the latter, it makes sense that he left Ctesiphon in order to rejoin the second Roman army/Armenians so that he could clash the coming Persian army in a pincer maneuver. It is possible that some of his commanders (both in the main and the secondary army) were not really up to the tasks;
The whole choice of embarking in an offensive campaign against Persia cannot be solely blamed on Julian. Constantius himself was making preparations for his expedition in the East before dying.