We designed the Bestiary to be a world save mostly so you and your friends could complete it together in one world. This also helped keep player save file size smaller, track progress easier, and enable Zoologist to be her best self. :)
Correct- you can fight EoC normal then swap to Master and fight him in that difficulty.
You cannot swap infections with the click of a button but it is now possible to build the opposite infection so your world has both.
Yes- you can fight invasions as long as you want. We extended it...
It is definitely possible solo with a fresh character.
You can literally dupe anything. You set the rules on how you want Journey Mode to work. If you think money duping is OP- just don't do it. God mode too op? Don't use it. Journey Mode is all about freedom to play how you want. :)