1. How do I get Preston to exit the Power Armor suit? I've read chat saying it's in the Talk menu, but there's no option for it there. He just keeps asking if I think we're ready to take the castle back.

2. How do I get to Salem and Libertalia from Sanctuary Hills?

The dialog with Preston gets kind of buggy between the points when Taking Independence triggers and when you actually take it back. With my first character I was completely unable to turn radiant quests in to him until after I completed Taking Independence. Is he an active companion when you're trying to do this? If so, try dismissing him, and then talking to him. He'll probably still do the same dialog asking if you're ready to take the Castle back, but if you talk to him again immediately after you might get the option.

I always recommend this map. Just type in Salem and Libertalia in the search bar on the left and you'll be able to find the locations.

http://fallout4map.com/

Do you have an active quest at Libertalia? There is one quest that takes you there, but it's kind of a late-game quest. It's not really a location that I'd suggest going to at a lower level. It is a really cool location though. It's one of my favorites in the game, actually.

To answer your 2nd question, I think the easiest path is to go from Sanctuary Hills to Corvega then continue SE to Wattz Consumer Electronics and then follow the road to Bunker Hill and go past it to the bridge leading NE and after crossing it you should reach County Crossing. From there go east to Revere Beach Station (give the Revere Satellite Array a wide berth if you want to avoid a fight) and then go NE and cross another bridge and you'll come up to Lynn Pier Parking. From there you can walk to anywhere in the NE corner of the map quickly and you shouldn't need to be at too high of a level since you're mainly going through the central and north central part of the map. Salem's just NE from Lynn Pier Parking follow the road. To get to Libertalia you need to go SE to Nahant Warf which Libertalia is connected to. Salem can get dangerous so I wouldn't recommend going there until your level is in the high 20s or so. Libertalia should be fine for a high 10s character though, perhaps even lower level if your character is well-equipped and combat-oriented.

or just bring a high range sniper, best supressed, and you're totally go.

stay stealthy anyway, they got a dude with a fat man.