So I’ve done a lot of reading on replication recently but one thing I haven’t seen anything about is what are the effects of replication while offline? Assuming you want an online and offline mode could the game act as the server and player? Or would you need to create entirely separate levels and actors for online and offline.
When playing offline your client has network authority over everything. So it is like a listen server. But its not a remote client at the same time (the BP node Switch Has Authority: Remote isn’t fired).
You don’t have to create separate levels at all. 
What would be the proper way to propogate client data to the server? (Necessary since my game can be played offline- obviously opens room for cheating but I can validate data to be sure it’s not out of bounds)
If you mean ways to reduce cheating risks, only send player input (like action events) to server, and only send info necessary for display to clients.
You’ll have to experiment with clientside prediction to mask that the client has to wait for the server to process the input and return the updated game state.
My game is an RPG for mobile, I want to have the ability for players to battle each other online. Unfortunately, since most of the game is offline, there isn’t really much I can do to prevent cheating. My plan is to validate the characters being sent by the client to the server to be sure they don’t contain any stats or data not possible by the game, so that worse case scenario, they just cheat on leveling.