All theories are a reach, but that doesn't mean they're impossible :D

Sure, but it matters whether or not it has a substantial basis, or whether you can answer the "why" and "how" properly

True, but I don't think we know enough about spirits to really say anything about "why" or "how" or substantial basis.

That's not exactly true, it depends on what your theory is about. I'm confident that my Demi Spirit theory has a substantial basis

I second that, it's the only available theory I know of that fits the story and involve plot holes.

Remind me what the Demi Spirit theory is? That Aiz actually dies in the dungeon, but gets implanted with a demi-spirit that allows her to live for a 1000 years until something happened to wake her up 9 years ago? Like a mass awakening of spirits? So what's to say this mass awakening didn't affect Welf's skill into resurfacing? To me all these theories are possible based on all our current knowledge of spirits. There was a reason why Welf was brought into that tent when the girls at the camp were discussing about Aiz and Aria, it may not just be a coincidence.

Theory about Ais

Most of the theory have numerous non-starter in them making the effectively impossible.

As for the time traveler theory? A refugee out of time would make far more sense.

While it is fairly obvious that Ais was sent or brought to the present via some mechanism or another, time traveler would specifically refer to someone capable of moving through time.

Uhh...what? I've never said anything like that. Theory is in the link provided by Kevhunt

Yes, and that was because they wanted to know if he knew anything about Spirits, which caused him to switch with Bell. You're trying to read into something that isnt there, they just wanted said information, nothing more and nothing less. Welf having his skill is just to flesh out his character as being different from his greedy family

I was just giving a possible reason why Aiz was in stasis mode for a 1000 years, still having memories of her parents even today. One idea being that it was something to do with the dungeon (which we know is an "infinite resource") and another being that it might be an inherent ability of spirits, there may be another reason based on where the spirits have been for the past 1000 years.

The thing with Welf may not be there, as you say, and this may just be a way for the group of girls in the tent to be introduced to Bell who read the original Dungeon Oratoria. However, I like to believe that the author was drawing parallels with Welf as a way of connecting the dots between him and Aiz. There is still a lot of mystery with how she came to be in the Loki familia but still retain memories of her parents. One of these mysteries could be related to why the Crozzo's got back their skill. Her connection to spirits seems to be much stronger than Welf's in any case since it doesn't seem he reacted too much when fighting the great spirits in SO12, while Aiz was shown to have headaches.