Yes it can do 2ml, just check it dispenses 2ml before connecting up your Heptavac.
When you push it against the animal, the front part collapses into the spring part and exposes about 1cm (-sh) of needle.
I wash and disinfect mine, store cleanly and reuse, albeit still only for Heptavac (no requirement for anything else, thankfully).
As for the holder thingy, it sounds like you've got the one with a long tube. I've moved to one where the bottle is mounted on the injector but still has the sterivax part. The one with the long tube I used to put the bottle in the chest pocket of a fisherman's jacket. So long as it's upside down, it will be fine. I suspect the bottle holder comes with the much larger bottle sizes.
Personally I found the long tube a hindrance, especialy with horned sheep, and it holds too much of the vaccine if only doing a few. I'd steered clear of the ones with the bottle on the injector thinking that I'd knock the bottle off, but not had any problems and much prefer them. I keep my old tube one as a spare.