Then I would definitely test at least one for frame, since it is definitely in the breed. It can hide, and no evidence of it being absent from pedigree should rule it out. If both parents are carriers, there is a 25% chance of a foal that will die.
LWO (Lethal White Overo) is only a threat in Paints, not Quarter Horses. If both the dam and sire are registered Quarter Horses, than LWO should not be an issue and a test should not be necessary.
As for the dun factor, that's always something to consider. If both parents are registered, you should be able to view their colour history/lines.
Do you not have a copy of both papers to research their lines? The AQHA will have the most updated records and should at least help you with some of the information you're looking for (ie. If the dam is by one dun, then there's a chance she is a dunalino). If not, All Breed Pedigree (an online website) can help (not always accurate) if both parents have been added.
A "dorsal stripe" and "dark legs" does not always mean a dun. A true dun has a darker mask (darker than the body colour), a dominant dorsal stripe and leg barring (like zebra stripes). Does your mare have any of those markings?
If you have any pictures to post, that's always the best to help you with your question