I am in the process of tackling this project and will post pictures in due course. What started as a simple job has turned into a huge and expensive hassle because of the location (and interference) of these drains. In hindsight I should never have started as the original installation was fine.
I FINALLY have all the pieces in hand and hope to goo them into place this week. I finished applying the Interprotect around the flange bolt holes today.
Wrt to your questions: (a) It seems to me that if you are experiencing massive flooding related to these drains then you probably have done major structural damage to the boat and have bigger problems. This is a very strong part of the boat and the weak link is the plumbing. Your seasonal inspection and maintenance should take care of it.
(b) My solution also uses 90 degree elbows between the through-hull and the valves. It was either that approach or a great deal of glass-work to relocate the holes. For the proper install, I have all pieces in bronze and am using the Groco flange adapter plates.
(c) Relocating the valves to the quarterberth lockers -- I wouldn't. The lockers are not very deep and I believe you will have clearance issues. And the valves will still be difficult to access during a voyage once there is stuff in the lockers. Plus you will loose storage space.
(d) If you must have instant access consider plumbing the drains to the bilge area forward of the engine. On my boat (#166) this would place the valves between the batteries and the engine. You will definitely be using elbows in this location. Actually -- as I write this I am already rejecting it. The levers on the ball valves are too long to swing in this area.
(e) For faster access to the existing valves consider installing a bronze or stainless deckplate in the cockpit sole. You can reach through it to close the valves (assuming they are not too stiff or are seized). However, I'll bet that you are just about as quick removing the bolts on the cockpit sole panel as you would be unscrewing the plate.
(f) Transom drains are a great idea but the transom is above the waterline so water will not flow out once the level reaches that of the drains. I recall reading something about a Contessa owner who fitted dinghy-style flapper drains so that his cockpit would empty more quickly if filled by a large wave. These freeing ports would handle the bulk of the water and the last few inches would pass through the existing drains. I always thought that this would be a good idea but I have not yet felt an urgent need for it. Improving the companionway drop-boards would go hand-in-hand I think.
Good luck with the projects....