Hi, Ron - this info. was included in an email to you but I thought I'd post some info. here in the forum as well.
Your Matilda Eberhard may be related to my grandfather Llewellyn John Eberhard, but it is difficult to say. Llewellyn was born in 1868, and Matilda was born in 1819, so she would be of the generation of Llewellyn's grandparents. They were both born in Lehigh County PA, so they could well be related.
Going back, my friend Keith and I found Llewellyn's father David Eberhard living with John and Caroline Eberhard in the 1850 census, and we assumed these to be David's parents. This John and Caroline Eberhard were at the same address in the 1860 PA census, living next door to their son David Eberhard and wife Catherine. (NOTE - Although, as you suggest, David's mother may have actually been John's first wife, Christian (Eberhard), who was with John in the 1850 PA census, with younger David in the household then, not yet married.)
So, possibly Matilda Eberhard is a brother to this John Eberhard, or a cousin. However, there were several David Eberhard's in the 1850 PA census, so another David, and another father John Eberhard, may be a relative.
As you point out, it is interesting that the "English-Welsh" name of Llewellyn was used in German families named Eberhard, both in yours and mine. And, I mentioned in the email that Llewellyn's brother was a Henry Eberhard born about 1855, maybe a connection with your Matilda and Henry Eberhard born earlier in the 1800s. Perhaps, there is an English-Welsh connection into the Eberhards of Lehigh County PA in the early 1800s (1800-1820) that has yet to be explored or discovered.