“Who you suppose this “great multitude” is?”
I dunno, do you?
You said: “They are the first fruits of the resurrection, now in heaven preparatory to serving as kings/priests during Christ’s 1000 yr. reign. (Rev. 20:4-6) So they could not represent all Christians but only those chosen to serve as king/priests in heaven.”
Are they alive in Heaven?
Yes, I do know.
As to your question, are they alive it would seem odd (a word you’ve used often) to believe they were in heaven and dead there seeing God is a god of the living and not of the dead. So.....of course they’re alive!?!