Regarding "pushing through a stopped pump". This is really only a bit of an issue with the closed-impeller (high-head) pumps. With regular open-impeller pumps, it's actually fairly easy to push water through them when they are stopped.
Basically it's a case of if you have two cheap-ass open-impeller style pumps, then sticking them in series is a good idea as even if one breaks down, it won't offer much resistance at all to the one remaining working pump.
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