The manual for my Bukh didn't mention antifreeze either (nor did the Volvo), both instructed simply draining the system, but I figure a 3 dollar jug of antifreeze is cheaper than re-plumbing the engine, so I do both. Do try to find out if there is a drain plug besides draining the impeller case. When I take the plug out water comes out, and that's after I've drained the impeller housing, so draining the impeller case isn't getting it all. But our engines are different. Seems to me my Bukh manual didn't mention the drain plug, it was the Bukh guru at Krinmar in Kingston that told me about it. You may want to ask a mechanic about that. Something in my dusky memory tells me Faryman became Bukh, so I'll tell you were it is on my engine and you can see if yours is the same. Here goes: Looking down on the engine from above you'll see a shape on the top of the engine that is a triangle with rounded corners. The apex of the triangle points to the stern of the boat. Find that shape, then look down the back of the engine below the apex of the triangle. If our engines are similar, there should be a drain plug about 2/3 the way down. It has a regular nut-type head on it about 12mm in size (it think). Back that out and water should follow. As the manual didn't mention it, it's probably not crucial - maybe the voids it drains aren't full so no threat of cracking etc. I feel better having it drained though! On the Volvo the frost plug was a petcock drain fitting in a piece of cooling tubing below the level of the bottom of the engine - one simply opened it and all drained out.
My water tank doesn't have a drain plug either, I just stuck the sucking end of the siphon in via the inspection hatch and let 'er rip. The drain hole at the bottom of the storage space under the vee berth is just low enough to let the siphon empty the tank.
As for a drain in the keel, I don't know if every model of Contessa has them. On mine, an '85 JJT, it's on the outside at the bottom of the keel (read ground level), way down low on the port side, aft end. It's a threaded fitting that you need a big flat screwdriver to remove. If it's there, take it out and leave it out for the winter to avoid ice build up in the bilge if you have any leaks.
I'm off line once I shut the computer down tonight, so good luck and I hope all is still working in the spring!