I can totally understand your fears regarding the remnant stomach. They may tell you you're fine and it's okay to proceed with SG. If not and they are recommending RYGB, then I do think you should discuss your concerns and family history of stomach cancer.
Obviously your doctor would be the best source of info, but I did want to mention that the inability to do an endo is probably not as big a concern as you think. There are lots of other ways to monitor for stomach cancer in this instance. It would also be worth discussing with your doctor whether you are at increased or decreased risk of developing stomach cancer currently vs. if you did have RYGB. It's entirely possible based on the endo results that failure to act may also increase your risks, so it would be a good idea to discuss all options.
I do also think it would be unwise to let this particular fear outweigh the real risks of obesity and the other comorbidities you already have. As I'm sure you know, with the issues you currently have your risks of a heart attack are quite high.
Best of luck with whatever you decide.