gallathan

nacd and nss-cds as well as iantd / tdi do cave qualifications. nacd / nss-cds seem to be most useful in terms of getting into cave sites as they're most recognised round the world. Some places offer dual cert. My understanding is that you're NOT OHE qualifified as mod 3. You can't do MOD1 in an OHE, but you MAY elect to do combined mod2 / mod3 and cave training...

Theres no such thing as cavern, intro, apprentice cave diver ccr.
Why - because Open circuit these limit you to single tank, 1/6 of gas supply, no stages etc. No such thing with the Rebreather, you could pass the course and be hours into a cave with the capability of most RB's. So you need a course about a week long and MUST do ALL the cave skills, right up to open curcuit full cave equivalent, and all the CCR bailout / use skills too, because when you qualify you can go as far into a cave as the Rebreather can take you.
With no prevoius cave experience, most are recommended to do the course open curcuit first to get the nav / line laying / comms skills etc, then do the course closed circuit. I believe the course can be done in about 5 days to a week if you're perfect at everything, ut you might want to allow 2 weeks, for re-doing dives, kit issues / delays etc. Jut my 2 cents worth / limited understanding