As for you're engineer turned soldier, I may be able to help there, depending on what you want.
Take a marine and make him able to build forts and roads.
Yes, but then he would be able to build farms etc and still have no attack, and anyway I want hings like a spade and a Royal Engineers cap badge so I can tell who he is.
Did you try to give them a combat-strangth? I think if you did so, they would be able to attack and defend.Engineers go armed, so prehaps he'd have a trait like the scout/explorer that he can't launch attacks. Neverthwless, they don't just surrender.
I would still appreciate a model though