A helpful person has pointed me at a paragraph in GMail's OAUTH terms and conditions that I had overlooked as I tried to work out what did and didn't apply to me. This paragraph appears to say, fairly unambiguously (which would be a first in this whole process) that because Pegasus Mail uses only local storage (i.e, it only ever stores mail on devices belonging to the user) it should be exempt from the GMail validation process and its associated fees.
I'm looking into this as best I can, given that Google do not provide any contact links of any kind to get authoritative answers from real people... But if it turns out this is correct, I'll restart the publication process and see if we can't get the code approved soon.
Fingers crossed, everyone.