“And that business with Thomas putting his fingers into His wounds and his hand into His side? What was that about?”
How best to prove to Thomas Jesus was alive? Angels had the ability to produce a body that could eat, be grappled with, able to hold hands with someone, be lusted after!, so why not Jesus having that same ability? Even if that body did look the same during one of his appearances.
Romans 8:11 says nothing about going to heaven or fleshly bodies going into heaven, note vs. 10 and the sense in which the body is dead and given life.
Paul says that if God could raise Christ he could raise those Corinthians from being dead in their sins. (vs 11).
“What about what was foretold in Isaiah 26:19?”
A resurrection is foretold but there is no hint of any heavenly resurrection being spoken of, a privilege not open before Christ, least of all an assumption of fleshly bodies into heaven.
In fact Isa. 25:8 was referenced by Paul (1 Cor. 15:54) and applied to the resurrection to spirit bodies and with that, immortality. That was not available before Christ so Isa. 26:19 would apply to those who would be subjects of God's Kingdom, not it's rulers.
It certainly makes sense that if Jesus's body was not raised from the dead, and He did not in fact ascend into heaven, neither do we have any basis for that hope.
This interpretation of yours, that it was not really His body, definitively rules out the Assumption of Mary and all of our bodily resurrections. On your account, this is impossible. You're consistent here.
The Muslims are even more consistent, in that the majority of them believe that God gave someone Jesus' appearance, causing everyone to believe that Jesus was crucified. Another (minority Islamic) explanation is that Jesus was nailed to a cross, but as his body is immortal he did not die of crucifixion, it only appeared to.
They said (in boast), "We killed Christ Jesus the son of Mary, the Messenger of Allah";- but they killed him not, nor crucified him, but so it was made to appear to them, and those who differ therein are full of doubts, with no (certain) knowledge, but only conjecture to follow, for of a surety they killed him not:- Nay, Allah raised him up unto Himself; and Allah is Exalted in Power, Wise.
—Qur'an, sura 4 (An-Nisa) ayat 157-158
They are consistent in their view that God provided a "seeming" body for Jesus, a kind of costume or illusion, and not that He is the Incarnate God who in the actual flesh died, rose and ascended.