Are there any air leaks coming from these areas? Is the same problem evident on the floor or just the tubes? Is it a bucket boat or a self bailer?
I am not 100% sure about your boat, but as I recall the old rikens have a full wrap of protective material on the bottom that comes up to the rub strake. If it is just the protective material and not the air holding layer or the bucket floor then I would not worry too much about it.
System 6 urethane works good (you will need the hypalon primer), but it is expensive and requires a significant amout of prep work.
In my experience, signs of rubber degradation mean that a raft is approacing the end of its usable life. You can paint stuff on it, or try to cover spots up with patches, but without a sound substrate to bond to these things will be difficult. I have seen quality urethane products stick well to bare fabric for a few years, but you need to ask yourself if the investment is worth it.
Mabye you could post some pics? I am picturing some cracking on the protective wrap right where the boat folds when you roll it up. If this is the case and there is no air leakage, then I would say just let it be and be happy that you have an old hypalon boat that is still in pretty good shape.