Abnormally-plumaged Pileateds mistaken for IBWOs in Texas

1. If you saw a good-sized black-and-white woodpecker with white trailing wing edges, was it an IBWO?

Not necessarily. Here's an interesting snippet from this paper, which discusses the potential for IBWOs in Texas:
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In the 70s there was a partially albino Pileated Woodpecker that was very much patterned like an Ivorybill. It required a visit. Already since the Arkansas sighting another bird like this has been reported in Baytown. Fortunately with the use of Internet birdcalls, this Baytown bird was identified as a Pileated without a trip to the home. These birds demonstrate that plumage alone will not assure you that you are tracking an Ivorybill.
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2. Ok, trailing white wing edges aren't enough. Let's say the bird also looked large, and it didn't undulate in flight. Now we can call it an IBWO, right?

Actually, no.

Regarding size, Tanner said: "...the difference in length is not a reliable character unless the two species are seen together."

Regarding flight style, Tanner said: "I have frequently seen Pileateds fly directly, in no way different from the flight of the larger bird."