Check valves are guaranteed to foul up and fail eventually, often within 6 months. All the professional plumbers I know in the aquarium hobby (one plumber) hates them. In the situation I have one pump down, I'll notice from my power drop eventually. In five years my main vectra l1 pump has never failed, so this is a pretty unlikely situation. It's not a problem if one pump pumps some water back into the sump. Turnover in the tank through the sump isn't as important a in tank water motion.
I was going to rig up a slick ball valve and union setup so I can swap pumps on the fly if it ever is needed though, because PVC parts are relatively cheap. I also liked my in sump salt mixing station so another ball valve that flows back into the sump directly will be there.
This was basically my approach as well. Some valves in key locations (including in between) to divert water wherever, including into both lines in the case of a failure or pump swap. If one pump goes down, it does dramatically reduce the flow and gets very noisy but it keeps the water moving which, as you point out, is the important part! In my case there’s a closed valve between so the good pump isn’t forcing water through the dead pump, but the tank IS continuously siphoning water back down through the dead pump…
My pumps are on distinct lines when running normally because of the valve in between, my impression from some preliminary couch googling was that two returns plumbed together would “fight” each other and create more head pressure. I have the luxury of using two return nozzles so I didn’t dig much deeper.