Interesting about the freezing bilge water. I have a wood boat and always live aboard. In the coldest weather the water got down to 40° with the air temps about 10° above and the bilge temps measuring 34°. I never had a freeze up.
Maybe Rule got tired of everyone copying their bilge pump design. The cleanout is handy, but any debris will hold it open. With a proper vented loop it should safe from siphon.
I don't live on the boat and was not running any bilge heaters or heaters. Sometimes I do put a light bulb in the bilge, put they can break.
My boat bilge collects rain water, which freezes at higher temp than saltier Chesapeake Bay water. I always have 2 inches of water in the bilge. Yes, would like a dry bilge. I think would require a diaphragm pump. That picture, boat was in the Back river near LAFB and it got exceptionally cold. I have had only a few times where bilge water freezes solid, more times will get a crusty ice layer on top of the bilge water, still the pumps wont work well with ice in the bilges.
I also have 2 separate bilges.
All my pumps are Rule, and they all thawed and worked fine. I have no pump problems with Rule, I have had lots of Rule switches fail. I have 6 bilge pumps. I have another used Rule 1500 which is not connected.
So forward bilge has Ruel 2000, Rule 3700, Rule 3700, Rule 300 (part of the shower pump).
Aft bilge has Rule 2000, Rule 500.
None of those could have worked when the bilge froze. That year we had a snowstorm dumped 2 foot of snow on the boat. And as the snow melts, some of the snow flows into the bilge. So when it melts and the bilge is frozen, the boat just has to cope one way or another.
Regarding the entire aft bilge, cockpit will fill with snow and all of that water will flow into the aft bilge. At least snow takes up a lot more space than liquid water.