If the horse is on constantly - or mostly - wet footing, this will cause hooves to be softer, weaker, prone to distortion, infection etc. Dry - or at least mostly dry - footing is best for horses' hooves.
If you've ever looked at trimmed sections of wall material in the summer, after a day or so, you will know that yes, dry hooves shrink, contract(which would make nail holes looser), and moisture keeps/makes them 'expanded'. But I don't think that's relevant, or the reason for your issues. For one, outer hoof wall is impervious to moisture, unless cracked, etc. And the inner hoof wall is 'moist'(more so closer to the laminae, less so further out) anyway. So wet footing will allow the sole & ground surface of inner wall to absorb too much moisture. Cracked areas of wall, and in nail holes will absorb too. But not deeply. IF it's about too much moisture, see paragraph above.
Not saying farrier is necessarily to blame, but I don't believe, just because horse has had 'history' of this prob rules out bad farriery in the least. Yes, I think poor quality hooves, maybe just from too wet footing, quite possibly from other factors too, including the way the horse is shod, is the probable cause.
Nutrition can well be a cause of weak feet - can't make a good cake without all the right ingredients! So perhaps the Farrier's Formula is a good option for her. (Tri acta appears to be a joint supp so not sure if it's got anything in it good for hooves) It's important too for nutrition to be well balanced, and depends what's in the horse's diet as to what she needs in a supp to 'fill all the gaps', so if you haven't best to do a diet analysis, then you can work out whether the FF is indeed an appropriate supp for her & whether it's providing all she needs.
Oiling hooves does nothing at all for their health & is potentially problematic, esp if she is in wet footing/there may already be infection present. It not only potentially softens hooves, but seals in moisture & infection... and hoof infections are almost invariably anaerobic - thrive in airless environs. So I'd save the hoof oil for cosmetic only 'prettying' of healthy feet only.
How is she shod? Conventional rims, or...? How is her hoof balance? How often is she trimmed & what terrain is she worked on? I ask because I don't believe that shoes - even steel peripheral rims - are necessarily a bad thing, but it depends, and especially with already unhealthy feet, they tend to be too compromising, just make matters worse. Therefore, regardless how she's shod, I suspect she would still be better off shoeless, at least until her feet can become healthy & strong.
Most horses need artificial protection/support for some situations at least, and especially if she has such weak feet, she will likely need hoof boots, or some form of artificial protection for some things at least, but unless she's really bad, she will likely be fine barefoot at home in her paddock though - should only need extra protection for some/all 'work'.
You might like to post some pics of her hooves here. If you do, please first take a look at the link in my signature line below for what's required of hoof photos.
Last edited by loosie; 05-31-2019 at 10:18 PM.