Here’s why your conclusion is (still) absurd:

You state:
I will not say I understand exactly and completely what Jesus is saying here. What Jesus means by 'fruit' and 'tree' is debatable.

You start by stating that you evidently are not sure what “bad fruit” really is.

Then in your conclusion, your logic then jumps to the fact that.....some sort of “Sin” is “bad fruit”. Especially Calvin’s sin. You called it “Sanctioning the burning of Servetus”. What would this “sin” be, asssuming the worst of Calvin? Complicity in murder? A sin of omission for not speaking out against “capital punishment”, which was the law of the land? Premeditated murder? Whatever it is it is some sort of “Sin”.

Then your logic..jumps again and presumes....1st he is guilty of “Sin” and 2nd he is un-repentant of sin before he “received his doctrine from the Holy Spirit” and/or after. (your words) Therefore..he is a bad tree. This logic also applies to Paul...he also is guilty of “Sin”....especially prior to his conversion...and didn’t Paul state “O wretched man that I am!”..... after his conversion......but your logic says “our Lord warns you to beware of him.”...because he is/has “bad fruit”....”SIN”.

Since your definition of “bad fruit” by default is some sort of “sin”..shouldn’t we also beware of “George Fitt”? I would wonder if there may be some un-repented sin..concerning the 9th commandment and John Calvin. In your presumption that Calvin was unrepentant...have you ever read any of the published prayers of John Calvin? If not here’s a sample for your perusal:

Grant, Almighty God, that though we daily depart from thee by our sins, we may not yet be wholly removed from the foundation on which our salvation depends; but do thou so sustain us, or even raise us up when fallen, that we may ever continue in our degree, and also return to thee in true repentance, and whatever may happen to us, may we learn ever to look to thee, that we may never despair of thy goodness, which thou hast promised to be firm and perpetual, and that especially while relying on thy only-begotten Son our Mediator, we may be able to call on thee as our Father, until we shall at length come to that eternal inheritance, which has been obtained for us by the blood of thine only Son. — Amen.

From what I’ve read of Calvin, I would imagine that not a day went by without a cleansing of the soul in humble repentance before his God. We would all do well to imitate it.