I hesitate to give a shortcut, because I really, really feel you read through all the multiplayer tutorials, but here is the main design consideration:
All game logic and mechanics should be handled by the HOST. All PEERS generally only send their inputs to the HOST. The HOST will act based on these inputs received from the PEERS, then SYNC the state of the HOST game/objects to all the PEERS, so that they all have the latest state of the game.
The other main stumbling block a lot of people have is how to get connected and the concept of SIGNALLING SERVER, GAMES, ROOMS, and who is a HOST/PEER. But this really is generally very straightforward and the same for most games, so if you follow the tutorial it should be very clear. I think people have trouble with it because they didn't really read the tutorial to begin with.
Regarding starting from scratch - that isn't true! We spend the most time when we run into a mechanics problem that doesn't work right away as we imagine. Once you've figured it out, you won't need to figure it out a second time. Also, you can copy/paste events. When your base game works, all you need to troubleshoot is the sending of messages and communications. It is much more frustrating when you have to debug multiplayer communications at the same time as your game mechanics.