Trade evolutions had been a pain point in Pokemon games ever since we're allowed to trade Pokemon online.
Current Situation
Today, in any Pokemon game (except Legend Arceus), not just PokeMMO, trading evolutions always involve a risk if you don't know / trust the other person. If you're just trading to add an entry to your Pokedex is usually fine, as the pokemon in the trade often will have little to no value. But when you're trying to evolve a Pokemon like Kingdra after breeding it to have perfect IVs, nature, etc. Then evolving it turns into a big risk as there's nothing stopping the other person to simply take your Pokemon.
I know that most of the time, mods will be able to identify these cases and ban the player at fault given enough evidence. But overall, in my opinion, this mechanic on an online game is a design flaw, it doesn't really add any relevant value to these pokemons or to the game.
Proposed Solution
In my opinion trade evolutions should work just like they do in Legend Arceus. This means we'd need to introduce the Linking Cord to the game and change the items hold by pokemon during evolutions to work just like evolutions stones.
Changing this mechanic would not only completely eradicate the risk involved in trade evolutions, but it'd also reduce staff load as they wouldn't have to investigate / address this anymore.