First, is there a way to turn off the led's on the RGB modules? That might expand my options.
No idea as I have never owned any RGB RAM modules. I'm not a RGB fan, so paying extra for that feature isn't something I personally do.
Second, will the option(s) offered by the RAM manufacturers for a given motherboard work... even though the motherboard manufacturer does not include it(them) in their list of approved memory?
Motherboard manufacturers usually only test a fraction of the available modules (usually from their internal testing before the motherboard launches to ensure users can find a stable kit).
However, RAM manufacturers are always adding, removing, or changing the modules they sell. If the memory manufacturers' website says their particular modules will work with specific motherboards, then I would trust that. I've always used their "RAM finder" utility when I bought RAM for my builds, and they have always worked perfectly.
If you really want to be super cautious, you can always Google the RAM module number and motherboard model you have, and odds are you will get plenty of hits on hardware tech forums or Reddit where users would discuss any issues when pairing those particular units together. However, this is largely unnecessary since the memory manufacturer (who knows all there is concerning particular modules) state that it will be compatible.