Sneaky asking that question when you’ve written a journal article about it. 🙂
My conclusion, which I came to prior to finding reference to your article, is that “son of man who is in heaven” best explains the other variants, and that there is more than sufficient external evidence to justify following the logic here. I’d love to get my hands on a copy of your article and check your reasoning.
I would note that I think it’s marginally possible, though I can see arguments against it, to tie this to verse 11-12, in which I hear a later voice, i.e. comment from the time when the gospel was written, rather than a quotation of Jesus. If that were the case, then the potential difficulty would not exist, as this would me no more than an affirmation of the ascension.