Good idea with keeping it canon-1 up until the branch off point. Most of the characters involved would be dead by old age anyways for the other stories so I don't need to worry too much about keeping them alive. I may have one survivor who is a long lived race and is a mentor for the plucky young heroes later on.

The UNT of current times would be more powerful as it keeps getting new members and technology for the forseable future, but it's the height of their power when they were at their most militaristic. So you could say the height of their most deadly period since forming together into the UNT. War with the most powerful empire does that to you.

To kill the Threat at that point would require so much damage to the planet that it would throw off the entire star system and have a chain reaction. As of the current Canon-1 time they are just keeping it contained rather than trying to destroy it. There's nobody left to save down there anyways unless people crash land there.

Wormy head with tentacle mouths like a sarlac pit from star wars? Just as the head portion. It's fingers might be able to bend and twist around unnaturally like a whip and have sharp nails that shoot out and tipped with poison. T-rex feet would make the most sense or it slithers around with spiky tails.