Interesting questions.
On your first question, I think JW's would say that those who are resurrected to heaven are created anew into spirit beings from God's memory. The JW's teach that all the dead are merely in God's memory, like a gigantic memory bank of everyone's DNA and personality blueprint I guess you could say, and when they are resurrected (to either heaven or earth) they are 'recreated'. There is no need for anything deathless in the body to survive for this to happen, JW's would say. I think the JW's view on this is pretty reasonable.
As for your question about earthly survivors of Armageddon, their answer would be that only those who are under the ransom can survive. JW's would say these ones are the Great Crowd who survive because of having white robes washed in the blood of the lamb.
What is perhaps a better question is why should everyone else who dies at Armageddon not receive a resurrection back to earth. What difference does the timing of their death make? It would be grossly unfair for God to resurrect billions who died before Armageddon but not those who die during Armageddon (unless they have committed the unforgiveable sin). But you make a good point though, which I think proves that everyone must die at Armageddon who is not covered by the ransom (BUT they will get a second chance by resurrection back to earth).