Nice model.
She is big enough to convert to RC easily.
I am no expert, but in answer to your questions.
1/ you will need external ballast, some sort of fin keel would be advisable.
Modifying the keel to take a fin with a bulb weight may be easiest.
You will get lots of advice on here about placement of the fin, if that is what you want to do.
2/ Sail control is not essential, I have sailed boats with no RC sail controls, the sails were permanently set halfway out.
The only difference is you will not be able to sail her as far into wind, and you won't get maximum speed at most points of the wind.
Also it is a bit boring, just point the boat using the rudder IMHO.
3/ I have seen complete radio strays, including sail winches, hidden under model fish boxes. The only give away was the sheeting (control lines) but once 20 foot out in the water they would not be noticed much.
4x AA size cells will give you 4.8v of 6v which should meet your needs, so fairly small, and give you several hours sailing
(I get 8 hours out of a set of Duracell on a 31" hull).
Bouyancy you may have to relly on the gods. If she is well ballasted, with a fairly long fin keel she should not heel over too far. Alsoby reducing your sail plan (fitting smaller sails) you can also reduce healing forces. Until you see how stable she is it might be worth only sailing her in light winds.
I suppose you could glue in some blocks of polystyrene disguised as fish boxes, or under a tarp, where they will not foul the sheeting.
A micro server for the rudder may not cut it. You will need to increase the size of your rudder maybe by as much as 200-300%. This is because the density of the water does not scale. You can design this as a balanced rudder to reduce the amount of torque needed to turn it. But as I said, I don't think a micro-servo will have enough torque to turn a rudder through water.