Best guns for AD are the default ones.
What i used to do when i used the ship for leveling sailors was use 1 triple and 3 doubles, that way i was able to put it at 25/42 knots instead the classic 24/40 doria speed.
And yep, u can experiment with other guns and u can put back the old ones, no problem.

Yes, you can get them back.

feel free to try around.

But be aware changing the main battery on AD is a bit like swimming with concrete shoes. You can do it. Its not a good idea tough.

Ya stick with the guns. The difference between 40 or 42 OH speed does not make a big difference on a bb with the dorias turing force. Dodge like crazy and use those extra shells to land some great salvos