My ability to make assumptions exceeds even my own wildest fantasies. This sort of thing happened twice before but the difference was that the cartridge casing had cracked along a join and would subsequently wobble on the pedal stroke tapping the chamber. This time, the cartridge seems fine externally, but I cannot apply the same pressure with my fingers. If the new unit solves the problem then I'll put it down to minced bearings. I'll report back.
Regarding the crank arms, there are three factors that led to my woe:
1. Suntour components in this price band (cheap) are poor quality. The threads on the crank arms are shallow compared to a Shimano assembly I scrutinized alongside last week. I was able to take my crank extractor tool and screw it in fully with my fingers. The same tool on my completely clean, untouched (at the time) threads simply did not engage, seconded by a puzzled mechanic in the shop. I thought that the Shimano threads were more forgiving of the tool but no, the mechanic's extractor also could not engage (and could with the Shimano). Hence my 'incompatibility' thoughts.
2. I suppose I ride the bike quite hard. I have a fairly moderate hour and a half daily commute up a few fairly long, steep hills. This must put quite a bit of pressure on the spindle. Cumulatively, I can imagine this will make it harder to separate the different metals.
3. Partly owing to the shallow crank threads, the (possibly) shallow lead-in threads on the tool and the slightly curved crank, it was difficult to line up the axial plane of the tool with the spindle's axis. (Wouldn't a guide shaft be great.)
The new tool definitely seems to have taller, sharper, harder threads. Again, 'harder' is a hunch as the metal is black. If I can find the old tool, I'll post a pic of each side-by-side.