1. What kind of backwater valve do you use and which is the most fool-proof
and offers an easy way to get to and clean in case of a backup ?
You do not clean it in case of a backup. You clean it before and/or after a backup so it will be ready for the next one.
2. Since it has been said that I need to be able to access the valve to clean it, I am confused because my plumber told me that I should never open it because the pressure build up in it will create 20 ft spout of sewage. It seemed a little odd to me that there would be so much pressure, but what do I know.
The "spout of water" will be as high as the water in the sewer that is backing up. But if the BWV is not sealing and the water is backing up, that is not the time to open it and try to fix the problem.
3. Is there some special way of opening the thing and cleaning it ?
Unscrew the top, (as long as there is no backup at the time), and wash it out.
4. My local plumbing supply house only carries an Oatey backwater valve, which seems to me the check valve kind.
It depends on what I was concerned about. I might use a JRSmith combination valve under certain circumstances, for example.
5. Is there any backwater valve you would swear by ?
We usually swear at them, not by them.
6. If you have to depend on a BWV to prevent flooding, just be resigned to the reality that someday it is not going to work, or at least work properly and have a backup plan, such as good flood insurance, in place.