depends on location it was caught in, it can be legal.
For example, you could have grabbed a wild area event from the network once, and never updated your Switch wild area news ever again.
Updating your game version won't change your wild area events. So for example, if you're on the Delibird event, it would be legal as long as it's cause in Galar Wild Area.
Also, the date doesn't matter: it follows your Switch date, and you could have set it to any date...
I explained it in the last line above :3
I don't wanna go through all the hoops to describe every situation.
Basically, just catch stuff you can get legally, and you won't have to worry ever.
If the mon is on a game save that can be dumped, it is automatically not legit.
I think the word you're looking for is "legal".
There are a few shinies that can happen with den manipulation (RNG or save editing)
1. Seed manipulation. If you manipulate the "seed" which the game uses to call the Pokemon's stats, it would appear Legal.
2. PID editing. Editing the PID can make the mon look shiny, however if you check closer to try to match all the stats generated from the seed, it may end up being illegal.
PKHeX doesn't check for this just yet because the seed -> EC/PID/IVs/HT/WT process takes a while.
If you didn't understand what I just said.. well, I don't know how to make it clearer. This isn't the right place to clarify that.
Your usage of "dumped into a save" baffles me. I'm guessing you meant restored into a save.
Dumping usually mean the extraction of data to create a file. Given you're asking about putting files into the save, it's not the act of extracting.
Officially, you can have 1 set at a time.
In theory, you probably could have 3 different sets at a time, (1 for Galar Mainland, 1 for IoA, 1 for CT), but I'm not fully certain it'll function properly.