Thank you, mario49! Actually, I read this article before posting my question. Also, on a related note, I intend to have the phone data backed up in iTunes prior to boarding the plane and losing internet.
But unfortunately, this doesn't answer my specific question. Clearly, by not being online...I will be at least temporarily disengaged from my iCloud account. When I click "sign-out" (at any point....online or not) is the resulting action merely local (my phone erases a cookie or something like that) or does it require an internet connection? If the results are merely local and "sign-out" does not need to transfer any data back to my iCloud account then I shouldn't need the connection, right? This assumption is what I am trying to verify.
While I could always hand over the phone and head to the nearest wi-fi spot to erase my data through iCloud, that doesn't seem ideal on many levels. Because I do trust the recipient (my mother-in-law), is this approach worthy of consideration as "Plan B?"