Hi Chris,
Fun to see more photos - you must have enjoyed being able to bond a bit with your new boat. Good times!
Like Tim said, those are nice, old-fashioned, bronze "cone" type seacocks. That big barrel has a slightly tapered "cone" inside it, and there is a hole that runs through the cone. When you turn the cone 90º the hole lines up with the hole in the boat and the hose, and you have flow. Turn it another 90º and the hole in the cone is not lined up with the hose or the hole in the boat, so you have no flow. Where the cone part comes in is that the barrel is slightly tapered, so that it makes a tight seal. You can adjust (tighten or loosen) that with the nut on the side opposite the handle. From time to time you take them apart to clean and lubricate them, and you can "lap" them if needed. They are nice units. Not as "maintenance free" as modern ball-style ones, but eminently serviceable and pleasingly husky :)
Spartan still makes that style of seacock, and they have diagrams that will help you get the picture, along with instructions for how to maintain them. You can find them here:
http://www.spartanmarine.com/source/27.htm
Also, Nigel Calder's Mechanical and Electrical Manual has drawings (that book is a must have, in my opinion).
They used to be made by more companies (such as Wilcox Crittenden), but I don't think anyone but Spartan makes them anymore (that I know of). (Oh, wait, Blake's makes some, but they are kind of sideways to the type you have).
On the chainplates: That's one of those tough calls. On the one hand, that one does look a little bit scary, and the bulkhead may have some moisture in it (or it may just be crushed a bit from over tightening of the rigging, especially as the boat was raced). So it might not be totally safe. On the other hand, if you start getting into rigging projects 70 miles from home, it could turn into a real saga, and not the fun kind.
Also, everyone's standards for boats are different. Ranging from "Well it's held up fine all these years" to "I wouldn't launch with that chainplate if you paid me." You have to find where you sit on that spectrum. If it were me, I would poke around a little bit, to make sure the bulkhead isn't seriously compromised there, and then think about stepping the mast and motoring home. At least then you are on your own turf. On the other hand, now the mast is down.... Decisions, decisions.... A lot to me would depend on what the facilities are like there vs. at home; whether you would be able to get the mast home un-stepped, etc.
Try to look at the rigging as a system, and make sure you have no weak links. If the spreader bases are cast aluminum (looks like they may be), give them a close look-over, for example.
As to whether I would sail a season.... hard to say. I would want to see it in person and know more. I would probably lean towards fixing it sooner rather than later, but then others are out sailing while I am fixing, so you may not want to listen to me!
That plywood "patch" is interesting. I would guess either a filled in through hull hole or maybe someone was going to mount something? Now you can play detective :)
Thanks for taking the time to post the photos - it's fun to see them.
Rachel
PS: Looking back at the photos, I see you have a gate valve on the engine intake, and maybe some unconventional hoses on the cockpit drains seacocks. Now if you go to make everything perfect you'll never get off the hard, but I would want to be sure that all of the seacocks are closeable, and that you know just how to reach them all, they all have handles (as Tim said), etc. Because I'm betting some of the hoses and clamps are things you will want to replace, and at least if you can close the seacocks you can recover from a problem with one of the hoses.
If you are going to be motoring, maybe take an extra good look at the hose on that intake, since you won't be wanting to close it underway. Gate valves are not ideal, but my former A-30 had them and when I took them out I found them in decent shape because even though they were brass they'd been in fresh water.
Likewise it's good if the cockpit drain hoses/seacocks are in good shape, because you need to leave those open at dock or anchor. Basically, try to make sure that you can keep the "sea"water out, and pay close attention to things that can possibly let it in.