1) How do I play multidisk games
You can change .DSK using the web interface (CD ON CPC) OR you can extract all .DSK's to the same directory and launch the game from there.
Ie.
|mkdir,"Turrican"
|dskx,"turrican1.dsk","Turrican"
|dskx,"turrican2.dsk","Turrican"
|cd,"Turrican"
run"turrican"
2) If I upload Parados rom in slot 5 and enable it with |m4romoff how I re-enable the M4 rom without deleting Parados rom?
If you use |M4ROMOFF, you simply powercycle the CPC (or use M4 reset) to bring back M4 rom. Any uploaded roms will not be deleted/changed.
3) Did I understand correctly that M4 can also act as memory expansion? If so how exactly?
No, it does not act as a memory expansion.