I'm sorry to burst your bubble love, but
If you used codes to get the item to unlock the event, its not legit, its not legal.
It's illegal. (besides, something isn't even legal, it cannot be legit)
Also, with the lack of Crystal and the GS Ball event in VC right now,
it is not possible to get any Celebi during normal gameplay,
(even glitches to obtain the GS ball is questionable, but right now Crystal isn't even out)
Important point:
I'm am not denouncing your effort. I think it is amazing that people have the patience to SR something until it is shiny.
But, just because you put in effort doesn't mean it isn't hacked/cheated.
The entire process has to be legit, for the Pokemon to be legit.
People put in a lot of effort to remove the shiny lock, to SR for a shiny Zygarde,
and it is impressive, but not legal nor legit.
[I put in a lot of effort to glitzer pop a Shiny Celebi, and it's illegal lol] Now, to go back to your question (on how to be legal and shiny concurrently), which I'll address from a VC point of view:
1. Even if Crystal is released, they could choose to not give away GS ball
2. Even if they decide to give it away, it may be Japan only.
If your Celebi is caught in a non-Japanese game, there is almost no chance it'll be legal, since only Japan got the GS ball event previously.
3. Wait for PokeTransporter update. If Poketransporter allows a Shiny Celebi to be transferred,
despite it not being obtainable, just proclaim it to be "glitched", not "legal", not "legit", and you get your wish.
I mean, isn't the most important thing, having your prized possession in Gen 7 through legitimate means?