Hi Tara and Michelle (welcome Michelle)
I think things may be getting a little muddled with Myrtle Ethel Sanders and her parents - who you believe to be William Sanders and Ethel McClure of California.
In your first post you state
The birth certificate we are now in possession of for this mystery child states the following: Mother is: Myrtle Ethel Sanders age 14 (b. Mar 18 1907 Douglas AZ) residence Miami AZ.
The birth certificate does not state a date of birth for Myrtle Ethel Sanders, just age 14, birth place Douglas Arizona, residence Miami Arizona.
The date of birth you state, Mar 18 1907 is the birth date for Myrtle
EVA (not Ethel) Sanders, the daughter of William Sanders and Ethel McClure of California, who you believe is your Myrtle.
Myrtle ETHEL Sanders, the mother of Florenda Lee Merrigan, I believe is the one born Myrtle Ethel ADAMS on
1 Nov 1906 in Douglas Arizona, daughter of J O Adams and Mattie Dunn who was adopted in 1915 by Albert Sanders (see my posts 36, 37 & 38 yesterday).
See also Sandra's post no. 25:
Myrtle Sanson
1 November 1906 passed away March 1996 - Last Residence 85502 Globe, Gila, Arizona. SSN issued Arizona (Before 1951)
I am not trying to pick holes in your research, I just don't want you to go off on the wrong track.
Any thoughts welcome

Jool