OK, so we have an anglicized family of Welsh extraction and we have a record that shows "Elizabeth verch John Powell." I recall a parallel naming format in the English Visitations of the early 1600's where you'd often have a Latin rendering of "Elizabeth, daughter of John Powell." So if I were wondering what to put for Elizabeth's LNAB, the first thing I'd do is see what the entry was for Elizabeth's father. If he were "John ap Powell" I'd figure the family was using Welsh naming conventions and list Elizabeth as Elizabeth ferch John, with "ferch John" as the LNAB. But I'd bet he was listed as John Powell, with Powell being the LNAB. If the latter, I'd make Powell Elizabeth's LNAB.
The one thing I believe everyone would agree on in this conversation is that "verch" is not a middle name.
In the Biography narrative section, a subsection on ===Name=== can be very useful in listing all the alternative names the person used or was known by. Then if one has erred in the data field, the sourced information is there with which someone else can correct the data field.