Great vids, that sparkplug looks lean burnt like you say but it also looks to have been exposed to coolant, if that plug was from cylinder 5 it could be that the head has been marred up a little bit from somebody reinstalling the head onto the alignment guide dowel on the block when installing the head, I've seen that issue a few times and have had it myself, coolant only leaks into one cylinder around the alignment guide hole location
You mentioned about coilovers on the rear? With the E30 rear subframe on the compact how did you manage that? As the spring is seated seperate to the shock
