Take it slow. How fast can you launch that anchor? One engine on your boat still backs better than my boat.
I take it the through hull for the bilge pump is above the WL usually? If not, I'd put it well above WL as soon as reasonable. Then you don't have to worry.
Check valves, vented loops, they can all allow the bilge pump to back siphon for one reason or another.
I'm enjoying your blog. It's sort of like a Ken Burns series.