1) Hard to tell, but I would guess that it will be fine.
2b) Given that you are all new players, I would just start the world at level 0.
2c) The world level gets higher and higher as you get further away from the starting area. It doesn't change as players level up. There is not a console command to increase the level. Each time you "win" a world and start a new one, it will default to a higher starting level.
The host can always start a new world at any time and increase or decrease the level to change the difficulty.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy