Ann -
You crack me up

.
That's a good thing.
Love the theory.
One thing though:
Clark mentioned Grandma Davis when talking about who he could dance with at the Corn Festival [I think that's the only conversational mention to this point]. Here he *thinks* about *Great* Grandma Davis. In conversation, I might call my Great Grandmother 'Grandma' [I'm not saying that's what Clark did, just that it's possible]. So although both Great Grandma Davis and Grandma Davis have been mentioned, it's *possible* he's referring to the same person.
I don't know if it's common around the rest of the world so Great Grandma here means Clark's parent's [either mom or dad] Grandparent [so Martha's Grandma or Jonathan's Grandma].
Maybe that sentence was worded oddly though - that's possible.
"one of the quilts Great Grandma Davis had made knowing she'd never see her grandson marry my mom."
So Great Grandma Davis wouldn't live to see her grandson marry Martha.
I see a couple of options here [there could be more, I'm sure, besides the theory you listed above, of course]:
A. Martha never married the grandson but has the quilt anyway [maybe he died while they were engaged or MIA and she moved on or... for whatever reason, they never married and she got the quilt anyway]
B. Martha *did* marry the grandson - given that it's a grandmother, she doesn't necessarily have the same last name as her grandson [maternal grandma - Clark's Dad's Mom's Mom - does that make sense?]
C. It could be a very close non-family member who is *called* Grandma. This could also be a... hyphenated last name situation like Lane Kent for Lois and Clark's kids. Or a second middle name for a very close family friend.
D. Maybe Jonathan was Jonathan Davis at some point and adopted by the Kents for some reason [but he still knew who his grandparents were].
E. Grandma could have been widowed and remarried and by the time Clark came along was no longer Grandma *original married last name* but now Grandma Davis.
F. None of the above.
G. It's all a big red herring to see what theories people will come up with

. And then the real answer will smack everyone upside the head eventually. A giant case of misdirection, if you will...
Or something.
More responses later...
Carol