You misread Junty, I said it was going to need to be
aimed 
ALSO NO TRIPLE ROCKETS!
Teambuildingkilling sentries would just be asking for griefing and a single newb engi could be a heavily disadvantage for his team.
True, just put up a sentry behind all the other sentries and see your team rage.
I like your first idea power, and tbh. I don't think the engi should have to return to his sentry to unsap: Constantly killing enemies while running from the front to the sentry is next to impossible.
Well there SHOULD be a drawback to it. If you just put stuff up and go on a killing spree it's more like you are different class entirely, with additional buildings bonus. And you don't have to actually kill anyone, just deal damage. If you are returning from the frontline to a sapped sentry, chances are you are going to at least encounter the bastard spy who sapped it in first place.
IMHO sappers should become timed. If the engi kills enough people to say.... keep his sentry 10 seconds alive, the sapper falls off. That should be very hard, but still reasonable. The spy can simply resap the sentry if no teammate comes for its rescue.
Good idea to think about, though it is unlikely for a spy to hang around for full 10 seconds to see if the sentry goes down - and this goes for every building as they all get repaired.
Better send that idea in, might as well get picked.
Meh... It involves copy and paste and fixing and stuff.
The repair-by-picking-up-boxes-idea sounds like a straight upgrade for me: It works the same way it works now, the engi simply saves himself the step of actually having to hit his sentry.
Well the drawback here would be, that you don't actually get any metal, so you can't build or upgrade stuff. I guess it would have to be some kind of an unlock "weapon"... but I like the shooting people idea more anyway.