I know that the best nature for Magearna depends on the set it is running. I wanna do some ou battles with it and I'm not sure I will be able to find someone who will trade me more than one nor do I have that many Pokemon that would be a worthwhile trade for it. So my exact question is, which one nature allows Magearna to do all its roles without severely compromising another? Don't mind about the evs since it's fairly easy to ev train and I have a lot of berries. Also, is it shiny locked?
I would say that the best nature for Magearna is Timid. Timid boosts Magearna's base 65 Speed to make it slightly less mediocre. This also helps the very common Shift Gear, Calm Mind sets that love to sweep. With this set, it helps with any Pokémon that manage to get a Speed Boost . The other set I've seen on Smogon was a Fleur Cannon with Volt Switch. Again, with Timid, the boost allows for a possible ability to outspeed an opponent, Magearna could escape a hit. Again, Timid is the best nature that suits the first set, but it certainly isn't a detriment to the second set. Another option is Modest if you want.
As for your second question, Magearna doesn't have a shiny form, but its alternate form, the Original Color, is shiny locked.
Source: https://pokemondb.net/pokebase/275211/does-magearna-have-a-shiny-form#:~:text=Regular Magearna does not have a shiny. However,,This shows that Magearna has an alternate form.
Hope this helps!