You have a mistake in your premise. Chabad Messianism does not believe the Rebbe IS mashiach, but that he can/will be. (Which is rejected by most since mashiach is a living person.)
In your question you keep saying J "was", but he very clearly was NOT the mashiach. To believe J was the mashiach is heretical, but to believe he might be [have been] is actually what this answer says.
However once you've reduced things to this level you have almost nothing left - J might have been, but history is filled with tons of people who might have been mashiach. There is little reason to venerate any of them.
And at this point, where J is worshiped as a deity venerating him is hugely problematic, even if he never actually did anything wrong in his life (which I'm skeptical is true, but I don't think there is much definitive historical record).
From Rambam Melachim uMilchamot Chapter 11, Halacha 4
If a king will arise from the House of David who diligently contemplates the Torah and observes its mitzvot as prescribed by the Written Law and the Oral Law as David, his ancestor, will compel all of Israel to walk in (the way of the Torah) and rectify the breaches in its observance, and fight the wars of God, we may, with assurance, consider him Mashiach.
If he succeeds in the above, builds the Temple in its place, and gathers the dispersed of Israel, he is definitely the Mashiach.
He will then improve the entire world, motivating all the nations to serve God together, as Tzephaniah 3:9 states: 'I will transform the peoples to a purer language that they all will call upon the name of God and serve Him with one purpose.'
If he did not succeed to this degree or was killed, he surely is not the redeemer promised by the Torah. Rather, he should be considered as all the other proper and complete kings of the Davidic dynasty who died. God caused him to arise only to test the many, as Daniel 11:35 states: 'And some of the wise men will stumble, to try them, to refine, and to clarify until the appointed time, because the set time is in the future.'
Jesus of Nazareth who aspired to be the Mashiach and was executed by the court was also alluded to in Daniel's prophecies, as ibid. 11:14 states: 'The vulgar among your people shall exalt themselves in an attempt to fulfill the vision, but they shall stumble.'
So clearly the Rambam says the at one time belief that J would be Mashiach was not heretical, but today it is.