I, personally, don't think that the Arsula/Ursula/Americas is your William's mother. Maybe there is something out there that connects them, but all I saw was the 1880 census, but that's just one name on one census. There's no bridge to 1900.
I think the key is Comanche Co. Here's the timeline: 1880 census: a James/Jim and William Mosley with their father, Sebron, in Navarro Co. TX 1896: a Charles Franklin Mosley is born in Comanche Co to William and Mary Mosley 1900 census: a William Mosley is listed in his brother's (Jim A.) household in Comanche Co. These two, per Jim's death certificate, are the same family as the Navarro Co. family. 1910 census: Your William is living in Hazeldell, Comanche Co. with 2nd wife, Addie, who has been married to for 5 years. 1940s (I don't remember the exact date...it's in my previous post) Jim Mosley dies in Comanche Co. and is buried in Hazeldell Cemetery.
What might be VERY interesting is to see if there is an obit for Jim. Might list William on there. And the courthouse in Comanche Co. might have some interesting things.
Could be a coincidence. I think your very wise to hold off until you are completely sure. Let me know what happens!
Good luck! Andrea P.S. One last thought. You might post about Charles Franklin on the Stephens Co. board. Might find a descendent.