1. There is a pipe connected to the shower drain which runs through the bilge space, having performed a few physical contortions you should be able to follow the pipes as the exit bilge/enter locker and connect to said bilge pump on the wall.
2. Yes, keep an eye on it, having said that unless I was ploughing through a lot of floating weed, twice a year emptying was enough (Solent and SW sailing) you may or may not have a red(usually) plastic spanner with a x piece cutout to fit the top….if you have one it will in your ‘what the hell do I do with these box’ that you’ve almost certainly started? They can be a pain to open but even a plastic strap ‘opener’ on the side should work. The interior basket simply lifts out for cleaning.
3. Yes and yes. Try yachtboatparts.com they do genuine VP filters as well as an excellent non brand alternative which I have used for several years.
4. Connects to the main bilge, essentially back up if the 12v pump under the saloon floor fails.
PS. Have you got the boat and separate engine manual? If not, we have them downloadable from here?