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B) you don't use another company's intellectual property or combine it with their's.

Does this mean "don't use both Gw parts and PP (or similar) parts in a conversion", or does it mean "don't use conversions involving PP parts (or similar) in any game or similar event arranged by GW"?

The first one.

Not that I really give a damn, but would the redshirts object to my Marines being on other manufacturers' scenic bases?

iIf they are the same size no,as they would be painted and unnoticable.

My local GW is normally picky about things like this and they're totally fine with my 3rd party bases.

They still reserve the right to object. Even if you use a third party base. According to their website, they reserve the right to take action against you if you alter the model in any way other than putting it together in the exact method shown in the instructions.

Including conversions involving only GW and GW parts.

That seems a little shifty to me.