JMO:

1. The external style will fit, but you may need to have the BB faced to get the sides perfectly parallel and square to the threading. Hopefully there's enough meat left on the BB to do this, since the width is kinda important, too.

2. Keep the long-cage Ultegra- the only thing a short cage will do for you is a slightly tighter lower run of the chain. Changing to a short cage would be primarily cosmetic, although it won't hurt anything.

3. Yes. Although I don't know where right off the top of my head...

4. Front derailleur- you may want to change to a double type- it will probably shift better. But the triple type will still work.