JGB - that's a lot of questions there
I've got the ARB CKMA12 High Output On-Board (not the twin).
Runs fine off the battery but works better if the engine is running.
I've got a dash switch (came with the compressor) and yes there is a monster fuse in-line.
The ARB compressors come with VERY DETAILED install instructions (almost a book
) but some of the instructions relate to air-lockers.
You can also get a twin that doesn't mount 'on-board' if you want (CKMTP12).