Reconstructing a ritual for reentry into Judaism isn't that difficult. Four components are mentioned.
1) "Recrucifying Jesus for themselves, putting him to open shame"
That would undo his resurrection and bring him back on the course to die the death of an unrighteous man, in the mind of the one undergoing the ritual.
2) "trampling the Son of God underfoot"
Which sounds like bringing Jesus under the subjection of the person and Satan whom he now serves, instead of serving Jesus whom all things are subject to.
3) "counting the blood of the covenant an unholy thing"
Jesus' shed blood which represents his soul are the most precious and holy things in the new covenant and also what is effective for salvation, so declaring his blood common or unholy undoes that.
4) "insulting the Spirit of Grace"
God's Spirit is mentioned several times in the OT but wasn't a huge matter in Judaism and they may have counted the Holy Spirit issued on Pentecost as a different spirit. Insulting him would undo any effects that that Spirit had had upon them.
A ritual like this wouldn't need to be particularly elobarate. Taking an effigy of Jesus and trampling him underfoot then nailing him to a cross then pouring out some blood and making a declaration against it and "the Spirit of grace" would be enough. But the effect that this diabolical ritual would have on Christians with an understanding of the elements involved would be devastating.
Speculative ? I think the components listed make perfect sense for such a ritual. It doesn't take much lateral thinking to see how that would work. I think it is much more speculative to first assume that they are all metaphors and then speculate on what it's all a metaphor for (sin, unbelief etc).