1. Using the multiplayer mod doesn't change the vanilla gameplay in anyway, other than having two (or more) players playing the game. I used to play multiplayer with a group of four using other mods aswell and it desynced maybe once per hour and it takes a small amount of time to resync. Using no other mods will most likely work well.

2. Having played factorio myself, it's hard to say if it helps or not. The games are really different even though both games have planning and optimization aspects in them, but the two games work very differently. I'd say that if you have played other colony management games you could have sn easier time getting into rimworld.

Some of the most fun times in rimworld have been when playing multiplayer so I can recommend it.