You are much too fixated on the quixotic notion of an ossified Pure Polytheist AntiChurch. A surviving European paganism would be isomorphic to Hinduism in many ways. It too would develop monistic and yes monotheistic <oh the horror> schools of thought. Actually at the intellectual level these would gradually dominate while the popular cultus would remain predominantly polytheistic. You have mentioned Julian. Yes he rejected Christianity for Paganism but he was also very much a Neoplatonist of the theurgical stream.
And in Platonism of any stripe the Ultimate is the ONE not the Many which doesn't sound very Pure Polytheistic to me.
The Neoplatonist crowd, for all their beliefs centred around The One, still respected tradition, and if they were to take control of the upper echelons of Post-Roman society, would still incorporate the identities and sacred stories of the Gods into whatever new ideology that would be born in the aftermath of the W.E's collapse, if it even happens at all. If not, there might still be an office or board of control created by the reigning Augustus, which might not exactly be the beginning of a new religion, but might take control and regulate all the temples and mystery cults within the Imperium Romanorum's borders.
If this began with Julian, and he were to survive and endure as Augustus for another couple of decades, the Christian Church may still be a major force in the regional politics of the eastern provinces, but they would still have a tough time re-asserting themselves on an imperial level, if Julian was successful in creating a central authority for all the Pagan cults. The Christians had also only enjoyed Imperial favour for mere decades when Julian became Augustus. Plus having cut funding to the Church and recalled certain dissenting bishops, not only could he have potentially saved the declining condition of the Army, but an increasingly factionalized and fratricidal Christianity may never again achieve their former influence over the Imperial family.
It might buy more time for the Empire, say until the 700's, but there would still be thousands of Germanic and Sarmatians in Roman military service after Julian's time. The re-paganized Roman regime, possibly with a reorganized army, might not be willing to pay off the Huns like Theodosius did, but the Sarmation, Frankish, Suevi, Vandalic, and Gothic element would still figure heavily in the frontier forces. Perhaps this situation might reflect the decline of the Abbassid Caliphate, where the Mamluks and the Turks serving in their forces had become increasingly powerful, that even though they were Islamicized, they still turned on their Arabic overlords, and began carving out empires of their own. Perhaps the generals of the Romanized Barbarian forces, having become so accustomed and inured to Roman ways for so long would have established themselves as rulers in several provinces and have taken the titles of Caesar or Augustus instead of just Rex or Konig, and would have retained for themselves wholesale Roman culture and law, instead of just tried to ape it.
The Post-Julian Empire might still fragment into the rival Imperiums of Hispania, Gallia, Italia, Germania, Britannia, Africa, Aegyptus, Asia, Graecia, Thracia, and Syria.