The first reason is that I could have sworn someone verified she applied for the SSN in TX around that time. LE says she arrived with the identity, which I assume she then made official in the state. She could have been living under that name for several years prior in another state - maybe the Scottsdale connection holds true and she was in AZ, maybe not.
Secondly, I'd like to know how LE is sure she arrived in 1988. Applying for the SSN at that time doesn't mean she wasn't living in the state for many years prior, right? What do they know that validates that year as her arrival?
I don't know about the noses...both are distinct, but it almost looks like Longview's nose hooks at a much different angle then MLD - but then again, the pic image of MLD isn't great quality. There is very little out there about Mary, including a concrete date she went missing (except the ambiguous "spring"). Kind of odd for a missing kid, isn't it??