I have to say this sounds like a really "crappy" situation (pun intended).
I am not familiar w/ check valves for a sewer line, but thinking about it, I can't imagine that they could possibly be very effective. I work w/ scientific equipment and use expensive check valves for clean gas lines that are only marginally effective. Get the slightest contamination in them and they don't seal. Just a guess, the main sewer line from your house isn't exactly a clean line. There's bound to be something get hung up on the check valve and prevent it from sealing. In that case, you're still gonna get back flow, just slowed down. If you have flooding for even 6-8 hours, that's going to let enough past to cause problems.
I suppose if the flooding is never as high as your foundation, raising the basement waste lines overhead would prevent backups. Then of course you'll have to add in a sewage pump that will be prone to failure at the least opportune moment.
Put in a pump that will try to keep things pumped out faster than the stuff is backing up the line. Not an attractive solution either. Maybe coupled w/ a check valve it would provide fairly reliable protection. Put in the check valve to prevent backflow. Then install a pump behind it. If things start backing up, the pump would kick in and force the water back out. Similar to the setup for a sump pump. Right off hand, I can't think how you could do it so you didn't have everything running through the pump during "normal" conditions when you don't need it, or necessarily want it. I'm sure there are some creative ways to deal with the problem.
I simpathize with your situation, however I am also glad it is not me having to deal w/ it myself.