Echoing the other responses:
A) While the RS tour documents say what luggage is "allowed," you can bring as much as you want, BUT you must wrangle it yourself.
B) Yes, you can bring your CPAP. I've used mine on our two RS tours plus multiple domestic trips and will, of course, take it on our third overseas tour next year.
C) Theoretically, you can take your carry-on bag and your personal item PLUS a medical device such as a CPAP on the plane. I did exactly that on our first trip abroad, packing the CPAP separately in the bag it came with. And it was a total pain the butt. Juggling three bags through airports, immigration checks, etc. was a mess. Here's what I do now:
I have the RS rolling carry-on as my main bag. I use the RS Euro bag as my personal item. In that, I put my CPAP, camera and stuff I want on the plane with me -- Kindle, earphones, gum, whatever. I printed and laminated a "Medical Device -- CPAP" tag and attached it to the Euro bag. But no airline or security personnel -- domestic or foreign -- have ever paid the slightest attention to the contents of the bag once through security. If I'm in a regular security line rather than pre-check I remove the CPAP and put it in a tub to run through the scanner, just like any large electronic device. Once on the plane, the soft-sided Euro bag squishes easily to fit under-seat, as long as you haven't jammed it to the max. And the Euro bag attaches to the handle of the carry-on so I can roll everything along.
Finally, I have taken to leaving the CPAP humidifier at home when I travel. While I use the humidifier nightly at home, I find I can do without it for a week or two if it means packing a little lighter. I did the distilled-water search on a couple of trips. Lugging a gallon jug down city streets and on the subway in Boston was enough for me. If you really need the humidifier, I think you're OK using bottled, filtered water for a week or two. The distilled water keeps mineral deposits from building up over time, but for a short period I believe you can just use regular bottled water.
Hope this helps. Enjoy your tour!