Safe? Nothing is safe. We're not the relevant staff for GF/ILCA/TPCi, we can't know for sure what they check or don't check.
If you're looking for guarantees (sounds like you are), we can't give that to you.
With all that said, I can suggest what could be edited, but remember that it is all at your own risk. It is based on information at the time of writing, and their checks can and will change.
A few simple guidelines:
1. Only edit things that can be "naturally changed" in-game.
2. Don't edit things that cannot be "naturally changed" in-game. 1. Only edit things that can be "naturally changed" in-game.
Things like Pokémon you have in your party or box. Or Items in your bag.
For example, levels of a Pokemon can only go up, not down. Items can go up and down, as Items can be received from trade (most of them), used up, sold, etc. 2. Don't edit things that cannot be "naturally changed" in-game. Don't edit things that cannot be changed. Like Trainer Name. Or Trainer ID.
As an extension to the above guideline, don't edit things that cannot naturally be decreased. Like play time (that can only go up, cannot go down), or Caught counter.
TL; DR: When editing stuff, whatever your edits are, it must look like it came from playing the game.
I hope this last line explains it all to you :3