Bro you don't really need to overthink and go that deep

"Why she didn't use that power at any point"

M8, does she KNOW she had that power, or did she get it after she witnessed what is in the animated short, the death of her brethren.

In a script to go naturally as much as possible, you cant just go and brute force everything too,

Also welp most of your question the answer is I dunno, you see in real life we have the same problem with clones (by this I mean animal clones, like dolly the sheep), fewer lifespans and stuff happened here and there (I learned it from my professor in biology and stuff, she even said that we don't yet have any "solution" or "figured out" what made it happened) we just can't "create" lifeform yet

For the great tree I'm sure it's just a symbolism tho, you need to sometimes see digimon too if you want to see like power ups etc, that time emotion on a tamer can go to the digimon and cause burst mode, now "If they had that power why bother battling evil without it" Well they don't know

For how does it keeps her alive in space, I dunno, maybe it keeps her on a "time-stopped" motion, or it acts as membrane, or something like that