I don't think there is any way to disable the credit system without making actual changes to the code. If you just want to disable it to push rare files better, though, it would be probably better to just up the priority of rare files further (like your second request suggests).
I thought the auto priority was actually based on the number of known (complete) sources, but apparently the wiki says otherwise. Not quite sure what the reasoning behind this is. Maybe the number of requests is a more reliable measure than the number of complete sources, because the info necessary for determining the latter is not reported by all clients. Another possibility is that this is for historic reasons, i.e. auto-prio was implemented far before the determination of complete sources was. But this is just uninformed speculation.
I can see how including "very high" as a possible auto-priority for files with no other complete sources could make sense, but this should probably be an optional setting or the auto "very high" prio should be somewhat inferior to the manually set one.