Petros,
I am not saying that you have to spend $3,000 on a boat to be competitive, but I don't think you can build a highly durable boat for $300, or even $500 in this size range. And a lot of people won't be interested in spending a lot of time building small boats if they can't use the materials to make them last. Just do a search on here for recommendations about building small dinghys, and you will find they almost universally include recommendations to sheath the boats in one layer of glass and epoxy. This actually detracts from performance, but it makes the boats much more durable.
The same issue comes up in protecting wood. Best practice is three coats of neat epoxy on all wood parts, not for strength, but for durability. Since this prevents the wood from rotting though it does add weight, and reduces performance. But I would do it to any boat I built, because I want the things I build to last.
I still don't understand why a boat must be retired from racing after it wins something. It doesn't add to the class, doesn't help other competitors, doesn't restrict cheating, all it does it punish someone for doing well. Also it is a bit unfair, if I win nationals once I have to eventually build a new boat, but if someone else gets second for twenty years in a row they can sail the same boat... What's the point, particularly if the plans for the boats are made freely/cheaply available.
I think you are right on replacing/repairing issues, as long as it is a one for one issue. My thought is that the boat as sailed must be build able for <limit, regardless of the number of permutations it goes through to get there.
I would be hesitant to make design requirements though, hard chines vs soft is an interesting design question, and since you are already creating a restricted development class anyway why add things that will sort themselves out anyway.
You might also want to look at the class constitution of the Moth, Aussie skiffs, and Finns, to see how other small home built classes write their rules. The moths for instance are a true development class that basically allows anything with a max leingth of 12' one sail of max size, and one person onboard. What they have developed into is amazing.