The FAA only requires the 19 hours cited by a couple of people. It would be very rare for anybody to be able to do it in that little time. There is no requirement for ground school and I did my study at home. There's also no written test for an add-on.
The rental situation you quote is unfortunately the norm. Some folks build time by getting their CFI, and teaching enough hours to get hired elsewhere. Another option is a partnership. We have 5 guys in the R22, and 4 in the R44. This has worked well for us and keeps the cost of flying reasonable.
Proficiency is another issue. There was a thread about this awhile ago on Vertical Reference. The poster made the point that what was really dangerous was a PP-RH who never flew with an instructor except for BFRs. I agree with him. That's why we require our pilots to fly with a CFI once a month, and then to do more intensive training (ground and flight) every six months. For the six months training we try to use a guy who is the best instructor I've ever worked with and is a DPE to boot.
As to Schweizers vs. Robbies, that becomes a holy war just like high- vs. low-wing in airplanes. Both are good helicopters. Schweizers are generally more forgiving about pilot errors. On the other hand, for someone who wants to teach, most schools use Robinsons. Robbies are harder to fly in that they require very subtle, smooth control movements. Once this is mastered, it makes the transition to Bells, for example, easy, because they're squirelly too. The helicopter guy at the Philly FSDO believes in training in Robinsons, and say that "If you can fly a Robinson, you can fly anything". There's a fair amount of truth in that.