American Heritage Dictionary of the English Language, 5th Edition:
coherent
1. Sticking together; cohering
2. Marked by an orderly, logical, and aesthetically consistent relation of parts.
incoherent
1. Lacking cohesion, connection, or harmony; not coherent.
2. Unable to think or express one's thoughts in a clear or orderly manner.
The reviewers of White object to 'incoherent' by stating it has ONLY meaning 2 above, and are oblivious to meaning 1. As to "multiple principles, relationships, or interests" I'm not sure what you mean by that. I think coherent definition #2 above talks about "aesthetically consistent relation of parts." It isn't a question of multiplicity, it is a question of aesthetic consistency and thus things 'sticking together' and possessing 'cohesion, connection, or harmony' (or the lack thereof). So I think what RE means by 'incoherent' is literally that, but the reviewers took the meaning #2 of incoherent and assumed that was the only possible reading he could have meant, and thus objected to the term on the basis of claiming it implies that a lack of adherence to a single GNS agenda literally implies that the game participants will be unable to express themselves clearly.
It simply appeared to be a rather narrow and ill-considered criticism by the podcasters. Lets give them the benefit of the doubt, you often stumble a bit when speaking live.
See above... this was simply the first dictionary definition that popped up for each word in a DuckDuckGo search. It is a commonly used and perfectly cromulent dictionary AFAIK (not being some great expert on such).
Yeah, I'd be interested to know if there is some great degree of divergence of opinion on these words. I mean, I certainly agree that people use 'incoherent' to mean "someone said something to me and it was impossible to understand." but that seems at best a specific variation of a secondary meaning of the word.