3 question that happen in the anime.

I don’t think it’s fair to say OP is nitpicking, they’re asking if anyone noticed something that explains the 3 points raised that they missed.
Personally, I just watch casually and don’t look too deep, but it’s a fair question to a source material reader with better understanding.

1. Pretty sure there's an explanation of when Andy revealed how her powers work by using his blood, it's just on a much bigger scale

2. It's a ufo design yes, but it's not actually tied to aliens. So even without uma universe, it can still exist (iirc it's an artifact so it's even less bound to the concept of the rules currently in effect)

3. There also was an explanation for this in the anime iirc, but basically, if a letter ends in "love, kuno akira" or something to that effect, it won't be able to be seen by anyone else. (I'm assuming this extends to even anonymous letters where it's obvious that it's a son writing to his mother but I digress.)
The reason he was able to write his manga was because he used a pen name, that's the gist of it

4. It's the same artifact that Unknown recreated with his pen, basically, someone in UNDER became 10x younger than how much younger rip became (if for example, rip became 10 years younger, said person became 100 years younger). So he basically regressed to before having that fatal wound

Ok so I went back to read the chapter (artifact was named soul caliber) and honestly, it starts to make more sense with the more recent chapters, though I'll try to keep it as spoiler-free as possible.

On a technical note, the concept of souls and ghosts in the UU universe is similar but there's a distinction I believe. Think of it as a venn diagram where all ghosts are souls but not all souls are ghosts (since they're in a living body and such).

And so, since fuuko is still alive when he pulls out her soul. It's still technically a soul.

(wait, I don't remember if the ghost uma has been mentioned in the anime yet, was it one of the rewards for the quests?)