To the best of my knowledge, Hobie warrants their kayaks to the Original purchaser for 2 years from the date of purchase, not the date of manufacture. If there is no documentation available, Hobie warrants the boat for two years from the date of manufacture to any owner, (not necessarily the first owner).
If you were not eligible for a free replacement hull, it appears that you bought your boats used or the problems occurred after your warranty expired. Nevertheless, Hobie usually handles such problems quite generously. If they're offering to replace your 5 or 6 year old Adventure with a brand new hull for about $1100 including shipping, that's worth considering IMO.
Perhaps a more important issue -- your old Adventure with the cracked drivewell nose is not repairable and not safe to use -- or sell; attempting to plastic weld this high stress area would only disguise the problem. Regardless of whether you and your son think this area is poorly designed, the hull is trash at this point and should be recycled so as to not pose a potential safety hazard to an unwitting or inexperienced buyer.