I have several 9x12, 3 with the aftermarket rangefinders. I have a Bee Bee with interchangable lenses (also a 6x9 with interchangable lenses). I don't find them that hard to use. Both my Avus have ibsor shutters, a surprisingly easy shutter to clean when needed. If you are going to be very precise then yes, you need to use the ground glass. Otherwise, the brilliant finder if it is working, or the wire frames, if they are still on the camera will allow 9x12 use quite easily. If using a roll film back, then masks would be best for the brilliant or wire frame finders.
9x12 film can be purchased, but selection is rather limited. I buy 4x5 film and cut it down. I have about 22 cut film holders. They are still available from time to time, but are getting harder to find, and sometimes more expensive because of that. The metal cut film holders were probably the best thing to use during their heyday, but do add weight to your kit. The again, who goes around fast shooting LF film.
I have never removed lens elements for closeup on a 9x12. I know that is done on some older LF lenses. I may need to check that out. Auxillery lenses were made for closeups if you wanted to go beyond the 1:1 allowed by double extension bellows (on those that allowed it). You could also get aux lenses for the equivalent of 35mm and 200mm lenses. I have a set. You can also get series VI pushons for filters, including IR. As mentioned, many of the 9x12 had both rise/fall, and shift.
Vegard dino - the choice if camera is very personal. But I think if you get a 6x9 or 9x12 folding bed camera, you will be pleasantly surprised at the ease of use and the quality of photos possible.
A 9x12 is almost 4x5, but is a much lighter camera to carry, and probably 5 to 10 cut film holders is all you would need at a time for most outings. If all you want one for is to use roll films, I would go to a 6x9 with roll film back, or just go to a 6x9 folder. I have a Zeiss 6x9 that is wonderfully light and compact, yet has that good Zeiss lens.
