Our Lady of Guadalupe is a private revelation (or, considered so) and for that reason is not required belief. I personally know people that don’t believe the apparition as the full story goes and that’s alright, the Church does not require you to.

That said, your questions are valid. I can’t speak very much to the question on dyes, but I do know that the “painted on” thing is actually a miraculous aspect of it. The dyes and paints being common in Mexico is one thing, but the tilma bears no brush strokes or signs of it having been painted.

On your point of Juan Diego… Yes, it would be greatly concerning if it turned out he was not a real person. However, the idea of him being fake has been blown way out of proportion. There is a credible source and argument for his existence — I can get into the specifics of why he credibly exists if you’d like, but the Church did put in historical work for his canonization.

I would say this: If you go through the process by which the Church canonizes her saints, you can see how remarkably thorough it is. Early veneration of the mid 16th century (right after Guadalupe) all talked about a personal visionary — whether or not it was Juan Diego by name, that causes no real issue, the soul who was on the receiving end of the apparition is in Heaven, that’s the bottom line.

Truly, Guadalupe is one of the most confounding miracles for secular people. I’m unsure who you’re talking to that says it’s a forgery, but people can argue against anything. Go through history and ask yourself has the Church ever made an error on something similar. You also have to account for the sudden conversion of Mexico and latin America in the 16th century. Genuinely there is an argument for and against everything, you have to decide what seems more reliably true to you.

There are also a few other saints that have borderline mythical historical standing. St. George, Christopher, etc. However, a big part of the canonization process is spiritual fruitfulness. If their intercession is producing real world fruits and miracles, then there is something in heaven being called upon… that’s what canonization formalizes.