Decent results James, and you are on the right lines now IMO anyway, but I would say that!! A lot of this is trial and error as you are finding but you will crack it I'm sure.
Not sure about the speckling effect you mention, I think I do see it very very close up but post sealing I tend to weather anyway, with maybe powders or washes so perhaps its not an issue.
Definitely try microsol and microset, see if that solves your backing problem. Another tip might be to try as best you can to cut away as much of the backing as you safely can to minimise any effect you see. A magnifying glass and a brand new scalpel blade helps with this task.
Finally, I see you masked the engine room windows with tape, I'd suggested getting a hold of some maskol for that kind of task/
https://www.humbrol.com/uk-en/maskol-28ml-bottle.html
an excellent product that goes on with a paint brush, and fills things like windows nicely for spraying and then just peels off as easy as you like when the spray coat has tried. Well worth having.