This seemed to be a perfectly reasonable footpath - has it been removed for new development, that imagery doesn't show?

ah okay - my bad. thanks for fixing them :)

Dont worry, it was only northern sections where the polygons mirrored each other exactly

you've added the tag building=yes to some large areas of land that are also tagged landuse=commercial. the normal convention is to only place the tag building=yes on specific/individual buildings and not large areas. the tagging of landuse=commercial is enough to let people know that there will be buildings in the area until such time that someone is prepared to map the individual buildings
osm.wiki/Buildings

Yeah, that was my struggle as well. I couldn't view the history or any changes related to it, and the only relevant page i could find was on boundaries. I see/have no issue with you changing the tags to fit relation:land_area as it does seem to fit better

It could be. I was primarily trying to revert a seemingly arbitrary change to type=multipolygon that was causing an issue in a renderer