1: You should be checking your nitrox fills directly before the dive as well as when you leave the dive centre, just in case a cylinder has got muddled on the way. It shouldn't happen if the marking has been done correctly, but why take the chance. O2 analysers are cheap, so yep, I would recommend buying one.

2: First of all, do you need Nitrox ready regs? UK standards state you have to have O2 clean regs if the O2 content is over 40%, anything below this, standard regs are fine, I don't know what the US standards are though.

Nitrox ready regs are sold/supplied as nitrox ready/O2 safe from the manufacturer (in the UK atleast). The first stage and second stage are green, as is the hose sometimes. If you've just bought standard regs, then chances are they aren't O2 safe, if you've used them with ordinary air, even more so. Getting an O2 clean on a reg set is also relatively cheap and easy, so if you're not sure, throw them in anyway.

3. Sorry, can't help there! I dive single 12 litre for recreational and double 12 litre for tech, no experience with anything else.