Since you are so experienced with horses then, and know so much about them, then you cannot possibly have escaped knowing the single most important and self-evident thing about the horse market, which is, VERIFY.
I still do not believe that a majority of horse sellers (or giver-away-ers) are deliberately lying. They just have a different interpretation of terms than you, the buyer, does.
Universal translator:
"sound" should not be taken to mean anything more than "I myself don't notice any major horrible limping most of the time"
"quiet" should not be taken to mean anything other than "I can think of worse things a horse could do, and this horse doesn't do those things"
"vaccinated" should not be taken to mean anything other than "according to the seller's personal principles of how often, what, and by whom, which may differ widely from your principles thereof"
"teeth floated" should not be taken to mean anything other than "we paid someone to put something in his mouth and wiggle it around a while" (i.e., there is floating and then there is floating...)
etc etc.
Plus which, on top of that, of course people often DO lie, and as horse people we KNOW that they often lie, and therefore why would anybody take anybody's word about a horse for sale?
And you know what, as a responsible horse seller (occasionally), I can tell you all sorts of stories about buyers doing the same sort of thing. So miscommunication is certainly a two-way street. For instance, for all you know, the horse's feet might've been a whole lot worse before the seller got the horse and they were feeling fairly chuffed at having gotten them into that good a shape. So might you in their shoes.
VERIFY. It's not a matter of trusting or not trusting, it's not a matter of people lying or not lying, it's just that different people use words differently and have different opinions and knowledge bases on which what they say will rest.
I mean, horse trading is the POSTER CHILD for being careful what you're doing and not taking someone's word.
The horse has clearly been better taken care of than an awful lot of horses in the world are being taken care of. Just because the previous owner wasn't perfect, or didn't do things the way you would do them, does not mean they are horrible people.
BTW, when a horse is undermuscled and bony along the topline that way, it is never a feed issue per se -- it is either the horse being ridden in an incorrect way, or holding itself wrongly due to problems such as saddle fit or bit/teeth issues, or the horse is in physical discomfort from other causes such as feet, back/pelvis, etc. So if her topline is an issue for you then that is the sort of things to look at for solutions, not the feed mill or supplement shelf.
Good luck,
Pat