Your horse's WLD may not be as bad as you think, so I would trust the farrier on this, he sees many more hooves than you and would know when to take stronger measures. Cutting large portions of the wall away is very invasive and potentially very harmful.It's basically surgery. It could actually open the foot up to worse infection than is already there, and it's possible the hoof doesn't really have WLD anyways.
Many people confuse a stretched white line with a fungal infection (it can look similar) when it's really just poor trim mechanics that caused the problem, or a previous dietary issue that's growing out. Horses on grass tend to get more stretched white lines. Why? The grass changes with the weather, so the sugar content does as well. This can cuase varying growth rates and mild, unnoticed bouts of laminitis. Look for growth rings and at each ridge is a compromised bit of white line underneath. If your horse is getting good trims, it stays in check and may never present a real lameness issue.
Stretched white lines are more apt to get fungal infections (WLD). Some horses have conformation in the coffin bone that actually never really connects well with the wall and is always a potential port of entry for fungi. It can be in just one foot or all 4. Also, if your mare likes to paw, it can stress the hoof more at the toe and cause more separation. Or if she has a sore shoulder or any problem higher up that causes her to land toe first on that foot, can cause more WL separation, opening it up for germs.
Removing large chunks of hoof wall would be like cutting your toenail back into the quick. The hoof wall needs to be there, not just to support weight and hold bones in postition (yes, the front of the hoof wall is directly connected to the coffin bone) but it's also a protective skin and cushion from rocks, debris, etc. It should be a last resort. If your kid got the yeast based diaper rash, would you cut the skin off his buttocks? No, you just put an antifungal cream on it and let the area air out some.
White lightening isn't likely to hurt your horse, but it can be irritating to sensitve skin, and most chemicals really have no business on your skin or your horse, but sometimes you need thses things to fix something. There shouldn't be enough to soak the skin of the horse.It works more by vapor action than actual soaking, so it should be below the hairline when you are treating.
A soak in an anitfungal prep. Does not weaken the hoof. And Occasional soaks in Apple Cider Vinegar can help, and won't weaken the hoof. Again, no matter what chemical you use, if you serve your horse a crappy diet and he stands around in wet soil, and you don't keep his feet trimmed by a competent professonal, the problem you see won't just go away on it's own. A proper trim would remove excess material that would interfere with the treatment getting where it needs to, but you would be suprised at how conservative that can be.
Be glad your farrier is reluctant to just carve away at those hooves. I'm sure if he has a good reputation, then you can trust he's not just being lazy. I don't do a lot of things owners tell me to do, cause they just "know" its a certain issue, but I do what I know is best, and it works out . After all, it's my JOB to know more about it than the average person.
Koppertox is not designed for WLD, you could pour it into the crevice and it's not going to reach the higher bits of hoof wall. And you should never soak a whole hoof in it.
Also, if it is a confromation issue or she's pawing at stuff, you will find it a neverending battle with a stretched white line. It could also be a clue to a a subtle lameness, and it's not necessarily infected. :)