I am pretty sure your problem has has to do with the gamedata and testgamemode not be overridden. Also, you should be changing a child of the original asset, not that original asset (if that picture has anything to do with the changes you are attempting in your mod). I keep sharing this around because I think it describes every step required to make a generic mod so see if there is something you haven’t done: Generic Mod steps for a MortarAndPestle - Pastebin.com. Obviously you don’t need to be making a new mortar and pestle, nor are having to deal with a place able structure, but focus on steps related to setting up PrimalGameData_BP, TestGameMode, and the Mod’s umap.
The reason why you see DinoArena specifically reference the dino BP is because of the scripts written in the event graph in the dino arena call that dino blueprint. TheIsland is able to spawn dinos through the DinoSpawnEntries blueprints.