Maybe a better question is: "Is it ALREADY proven?" I hate to even "go there", because this stuff can be so tricky. Just when you think there's no other possible way the pieces can fit together, something else shows up to throw you in another direction.
Before we get into that, I should say I finally read through that X-DNA article thoroughly. What I got out of it is that the cM of the X-DNA match means nothing - it can be all over the map for even one generation. What the match, itself, tells you though is that your common ancestor has to be one that you possibly get X-DNA from, and the same for the person you match. In your case, for example, it proves that your paternal grandfather is one of your ancestors who CAN'T be a common ancestor. This is what Helen as talking about, in another answer.
So what do we know so far, within reasonable doubt?
* Chris is a 1C-level relation, with an X-DNA match, no matching ancestors
* Your maternal uncle is ALSO a 1C-level relation, NOT a biological uncle
* A 1C-level relation can be a 1st cousin, half-aunt/niece, great-aunt/niece, or great-grandmother/daughter
* Chris and your maternal uncle are NOT related
* Chris was b 1949, her father b 1908, while you were born 10 yrs after Chris; your grandparents were born in <1904, <1910, 1907, and 1906; your maternal grandfather died in 1940.
We can deduce:
* We can throw away the gt-grandmother possibility right away, obviously, since you're only 10 years apart in age.
* Your youngest gt-grandmother was born in 1887, 62 years before Chris was born, so Chris can't be a gt-aunt of yours either. I'm just going to go ahead and assume you're too young to be one of her gt-aunts, too - I'm just including this unlikely case for completeness.
* So that leaves 1st cousin (1C) and half-aunt/niece as the only possibilities. If she's a half-aunt, the common ancestor is one of your grandparents. If she's a 1C, you share a pair of grandparents. To be a half-niece, you'd have to have a half sibling at least about 25 years older than you, so we can rule that out.
* Summarizing so far, EITHER
(1) one of your biological grandparents is one of Chris' parents. We can exclude your biological paternal grandfather (whether he's your "on paper" paternal grandfather or not) because of the X-DNA match. We can also exclude your "on paper" maternal grandfather, who was deceased when she was born, but your biological maternal grandfather could be somebody else (that's actually our best theory).
OR
(2) your paternal grandparents are also her grandparents - it can't be your maternal grandparents, because Chis doesn't match your maternal (half-)uncle.
SO... to answer my own question, no I guess we're not "proven" yet, but we've narrowed it down to a very small number of possibilities, and it should be fairly easy to narrow it down still further:
(A) For Chris to be a 1C, your father would have to have a sibling who had a child that you didn't know about. To eliminate this possibility, we just need Chris to tell us that she definitely has DNA matches to reasonably close relatives (say, 3rd cousin or closer) on both her father's and her mother's side. That would prove that her parents are really her biological parents - and that she CAN'T be your 1st cousin. Hopefully, that's an easy one
(B) With the 1C possibility eliminated, it's a matter of identifying which of your three possible biological grandparents are a parent of Chris'. The procedure is basically the same. If you can identify a DNA match with a 2nd or 3rd cousin who's related to you through your paternal grandmother, then that grandmother is confirmed (and should NOT match Chris), and you can eliminate that possibility. Do the same with your maternal grandmother, too.
(C) If you can do all that, then the theory is already proved, but for good measure try to also show that you both have matches to relatives of Chris' dad.
(D) We're assuming that your maternal uncle at least has the same mother as your mom, since the family doesn't seem to be aware that he's anything but a full brother to her. So they LIKELY had different fathers (rather than different mothers). Technically, that could be true without your maternal grandfather not being your biological one, so it can't really PROVE anything, but it's a strong "hint" as to what is likely going on.