I want to ask, can I use all assists or I can't use any assists? Also, can I have any difficulty or 110% always?

You can use whatever difficulty or assists you want to. Entirely up to you

And what about the weather? Because I did a 3 lap Short Season run (without video proof) and in Japan and USA I got rain, which meant I had a time which was much slower (16:21)

There's an option in one of the settings windows that lets you toggle weather, set that to clear and you won't get rain

Ok cool, thanks.