For your first scenario, I would not change it to a find since you can never know if you would have found the cache even if it was there. I personally would leave the DNF but there may be some argument to deleting the log altogether.
For the second scenario, if someone contact me about one of my caches what did not require any type of special retreival, and they described the location and circumstance that lead them to their decision not to retreive the cache, I'd let them log the find. That said, I have a couple of caches that can be seen, but their retrieval is half the challenge. For these seeing the caches would warrant a log in my book. I'd suggest e-mailing the owner with the circumstance and see what they say.
My last two cents is there too much sensitivity about posting DNFs. They help owners ID potential problems with their caches, and there also can provide alot of entertainment to boot. See the recent example from this cache.
Bootlegger's Lair