Hi again Cathy!
Well, I have some VERY interesting (I hope) things for you. It is not what you expected, but please keep an open mind! And I apologize in advance for the length!
I went back and looked at the 1910 census (the original image) and it seems pretty plain that 1)Ola W. is the wife of (it says "wife of") the head, she's 24, it lists the length of time they have been married (7 yrs) and she's the mother of 4/4 living (and of course, there are 4 children listed). Now, we all know census takers can do some wild things, but I REALLY don't think this is a sister-in-law. I think it refers to the wife...and it SURE looks like her name is Ola W.
So then I decided to prowl around. I looked at his WWI Draft registration. Several major revelations on that piece of paper! I know this is him as it mentions his "withered" right leg. And his address in N. Main in Belton. Important stuff: First, he is listed as Matthew William Wallace and MOST importantly he SIGNS as Matthew William Wallace. That switching of first and middle names I have run across before, and will be VERY important in a minute. Second on the WWI DR, he lists his date of birth as June 25, 1882, not 1883.And he lists his next of kin as Miss Ellen Wallace, N. Main, Belton TX. The Draft Registration was Sept. 1918, so Ellen was back with her father by then. And his wife is dead (as his eldest child is next of kin).
Hmmmm. Made me very curious. So I prowled around some more. One thing that I found interesting was that in 1910 Matthew William/William Matthew (who I will shorthand as MWWM) lists his father as born in North Carolina. This late in the western movement it's a little odd to have NC pop up (completely different from, say, 1870). What I found that was interesting was a William W. Wallace born in NC in 1910 census in the same JP precinct as MWWM. William W. is 60--just the right age for MWWM father to be. And in looking at the original, his wife is named Ellen (gee, does that sound familiar?) and they are with their daughter Eva B (20)Vivian (14), and Josephine (11). Everyone but William is born in TX. And get this...Ellen is the mother of 14 children, 8 living (so there are 5 more children wandering around somewhere), Ellen is 49 and she and William W have been married 34 years, so since MWWM is 27, she could definitely be his mother!
Now it bothered me I couldn't find them yet in 1900. But, hurray for Heritage Quest! Sure 'nuff, I found William W. and his wife, Ellen, in Ft. Bend Co. TX (near Houston, Harris Co. TX), in 1900. AND GET A LOAD OF THE KIDS:
William W Wallace 50
Ellen Wallace 38
Mathew Wallace 17
Dan C Wallace 12
Eva B Wallace 10
Clara M Wallace 6
Jesse V Wallace 4
Jossie Wallace 1
There's Eva and Jesse V. is Vivian (note the shift to middle name) and Jossie is Josephine. And of course, Mathew is MWWM. And since the 1900 census lists month and year of birth: TA DA! Mathew is listed as June, 1882. There's also a George Wallace just a few line up who is born in 1877 with a father born in NC, so I bet that's an older brother of MWWM.
So...there's some major rethinking to be done. First, MWWM's mother is NOT Ola, it's Ellen. Your grandmother was named for her, all right, but it's the Ellen, not the Leona. That puts the first name Ola back in contention for wife's name. And I'm guessing you will find someday, someway, that it was Ola Leona or Leona Ola (almost sings, doen't it?). Maybe Oleona, with Ola for short? The daughter that answered the death certificate questions may have gotten confused between "who is HIS mother" and "who is YOUR mother."
I am guessing you have already seen Evaline/Eveline in the Buckner's Orphan's home census in 1920. Ellen is not there. If you have not seen it, let me know and I will send that to you.
Your next major task, I think, is to try and find MWWM and Ola Leona's marriage license. Since they say they have been married 7 years in 1910, I would guess 1902/03. Hopefully, it's Bell County.
Finally, the polio thing is not really making sense to me. Certainly it is possible, but polio before 1918 is VERY rare...it's a mid-20th century disease. There were no polio epidemics in TX before 1910 (there were barely any anywhere). The Wikipedia article is pretty good, but if you want info a bit more scholarly (and dry) about locations and dates check the National Institute of Health:
Polio is also more of an urban disease...and Bell Co., and Ft. Bend Co, for that matter, were very rural at that time.
Well, hope this breaks down some good old brick walls!