Well, if you wanted to promote him I guess you could pretend that he was being sent up to a flotilla-level engineering position, but yeah I'd rather keep a good LI who knows my boat than start out with another - you'd have to spend renown to get an experienced one.
I don't suppose it's possible to promote him AND keep him... if he's already an Obrlt... I don't know if the game will let you have a Kptlt as an LI. I don't know if it's possible through Commander either, I've never gotten far enough to start promoting my officers!
Re the custom emblems, you can do them using the Turm Your Way mods, there are different ones for the 7B and 7C plus a fix for a small issue with one of them, can't remember which. Do a search in Downloads with keyword 'Turm' and they'll show up in the list. Mine looks like this:
I had to do a bit of extra fiddling because the template has a silver/grey background and I'm using the Black Boat skin, it was just a matter of making sure the emblems would blend properly into the skin's coloring and not the default paint job, lol.
I need to start a thread about that silver "patch" that still shows up on the deck with this skin - I don't think it should be there but I've looked at the tga files that came with the skin and can't see anything obvious (to me) there that's causing it not to get the same paint job as the rest of the boat. The skin's readme says it's for both 7b & 7c but I wonder if there's a difference between the two that's making the 7b turn out that way.