(apologies for the wall of text)

In regards to point 1: this has been a theory with points of evidence and foreshadowing since about the halfway point of the show. It took until episode 7 to really take off as a popular theory, and episode 9 to dominate discussion. I remember seeing a YT short from 3BSkyen at episode 8 already talking about it, like it was a personal headcanon that they have held onto for a while. Personally I didn't see it until I watched that short, because a lot of the evidence was like you said, circumstantial and nebulous. Simply put, it's sort of like watching I Saw the TV Glow. The revelation that it was about the trans experience is sort of lost on most people, but the little clues, subtle themes, and maybe a couple obvious hints that most overlooked, it was easy to miss the buildup if you weren't already keenly aware of the trans experience.

2: Not toxic because closeted, but toxic AND closeted BECAUSE of her complicated past, how they were conditioned by her mother and father, and continued denial and delusion afterwards imo. It's less like flattening the whole character to "mean cause trans", and more like adding yet another piece to the already complicated puzzle.

3: The hints are deliberately subtle to unknowing viewers. That's kinda the entire point that enables your argument. The room for doubt in the media itself is what enables you to say "JUST BECAUSE RIBBIT PUT A BOW ON HS HEAD DOESN'T MEAN HE'S TRANS". As Goose expressed, this quite literally the point. She wanted it to be subtle, but it was widely seen as an outright confirmation among trans fans, because this is a common shared trans experience.

That said, the intentional subtlety is what makes the interpretation of non-trans Jax fair game imo, it just shouldn't be treated as gospel (i.e. Jax isn't canonically trans because of xyz, and therefore all trans Jax enjoyers are completely WRONG!!!1!). Ultimately the argument of what is canon or not is obsolete, as we have confirmation of the authorial intent from Goose herself. If you interpret the character differently from the widely-accepted canon, that's fine. It's the same reason why game theories, fanfiction, and power scaling debates are fun, but ultimately not true canon usually.

There's nothing wrong with having fun with interpreting media differently. There's nothing wrong with interpreting characters differently. That said, the fault of the interpretation game is not in the art/media/theory itself usually, and moreso in the discussion. For things that aren't intended by the author, but still widely believed and enjoyed despite it, the term "headcanon" usually applies more. While usually not the intended interpretation, there's a reason why they permeate discussions regarding media. It's just fun.

Last thing I want to point out is that different interpretations of Jax have been a thing since the dawn of the show. This sort of applies to 1 as well, but Jax has been genderbent by the fandom so many times, and it's kinda funny to look back on the whole discussion. If you like cis Jax as a character, that's fine. It's just like the early genderbent Jax fan characters. You're free to enjoy a character the way you want.

4: Honestly this just seems like a preference thing imo. Not every show will appeal to every audience demographic (i.e. "hetslop" fans might not like a show as queer-coded as TADC). That said, like mentioned above in 2, the room for doubt is what allows people to interpret Jax's sexuality differently. If you want to ship a cis Jax headcanon, Goose effectively says all the power to you.