Mechanics and Fixing Any Questions Answered

the spinning should be in the direction of my pedaling - so surely it should feel almost like the float you get when you're between engagement pawls on a freewheel, rather than a resistance which jams the wheel?

Not necessarily, pedalling can tighten the cog against the hub leaving it some space to undo until it hits the lockring and jams again and it can tighten itself thoroughly enough against both the hub and lockring that the movement isn't immediate. Like, it'll jam so tight against the hub that you can get a bit of resisting the pedals before it breaks free and then conversely it jams so tight against the lockright that you get a bit of pedalling before it breaks free.

I don't know that this fits with what you are describing though, when it's happened to me it's a rather soft feeling, what you are describing sounds harsher.

Looking at the photos you've posted 2 things strike me,

1. There's a bit of paint loss on your chainstay around where the end of the spider would pass, is it possible that the crank arm (or a bulkier chainring nut) is contacting the chainstay here when you're riding and get a bit of flex?
   
   

I don't think this is very likely btw as the paint loss doesn't seem bad enough for this.

1. I think this one is more likely, could the top/front of the mudguard be jamming between the brake caliper/brake bridge and the tyre?