I know some (not a lot) of navicular disease horses who literally can not walk without their wedges. After years , even though they have minimal bone pathology present. WHY? Because the natural angle of their coffin bones was negative naturally due to weak digital cushion, and will not ever be correct without the wedges.
OK, firstly I agree it is frequently an effective & sometimes necessary palliative, which can make an otherwise lame horse comfortable. Which for one reason or another, may be the best option. Secondly, seems like we may be thinking about different specifics & I agree there is a big difference between wedging to *correct* neg angles and in 'jacking up' heels to 'relieve' navicular, over here at least. It's commonly done regardless of P3 angles. When it stops working, the hoof is jacked up higher, which will often give further relief for a while. When it's deemed they can't be raised any more & the horse is again hurting, denerving is then considered. So far as I've heard, it is not considered a temporary measure which can aid improvement. Not uncommonly, this wedging causes the horse to effectively suffer mechanical founder.
Even with need to correct P3, I think it's better to be done with pads/boots *if possible*(I acknowledge not always adequate, practical...). Because the premise that a weak DC cannot be improved is incorrect IME, but I don't know how much it can be improved with shoes/conventional wedges. I have a fair bit of experience with horses treated as such, but no experience of improvement in the state of affairs. 'Navicular' is generally still thought of over here as incurable & degenerative & if conventionally treated, that seems to be the reasonable prognosis.
IME neg or ground parallel P3 also tends to go with crushed heels(walls & DCs), of which are best relieved, so padding under the frog but not walls is helpful IME. Because to grow strong it must be used. The catch is, to be used it must be comfortable, but IME frequently they can be with frog support pads.
Does this foot look all that unhealthy to you?
I don't get the point at all of asking me to guess how long horse has been shod like that - it can only be a stab in the dark. Assuming by your comments it's a long time, so I'll take a punt & say 20 years.
That hoof looks relatively normal IMO. Not at all terrible but not great. I'm assuming the shoes didn't cause the narrow heels in the first place. I can't see the pics ATM while typing, and it could of course be camera view that gave me the idea, but I wonder about her toes being a bit stretched & toe sole/frog apex appearing quite shallow? Again, may be purely camera angle or the way horse is standing, but in the shod/wedged pic, she looks to have quite high heels, if we're talking about wedging to balance P3(?).
At what point do you decide you need a pad? Why choose a pad over a bar? How high would you go with the wedge before moving to the next step? Is it ok to wedge one hoof over another?
I would, as previous discussion, consider pads &/or *frog* wedges, when there are weak heels, discomfort without, &/or P3 misalignment. I would consider conventional wedge pads/shoes, in the above situations, when it is deemed a temporary or permanent palliative measure is necessary, or due to circumstances, other options for the horse's comfort weren't possible. Over here at least, they do often choose a bar shoe as an early measure for 'navicular' pain. I think this is due to reduction in feeling & covering/taking the weak frogs/heels out of commission further. When that ceases to be effective, the heels begin getting jacked up. Bob Bowker's theory of why this generally works temporarily is that it shifts the point of pressure in the centre of the hoof slightly.
Is it OK to wedge one hoof over another? Yes, if only one of a pair is neg angle, then it's important to correct this. But whether it's 'natural', such as a clubbed hoof, or made like that with high heeled shoes, I think it's generally best to consult a good bodyworker too, if considering/making changes.
If you go with a barefoot attempt, how many months could it take to see if it will actually work?
Depends on too many factors to say really. Depends whether you're talking keeping the horse completely bare or using padded boots or such, depends on your environment, depends on damage & age of horse, etc. But generally, you should be able to see at least some improvement immediately or very soon, whatever your approach, or IMO you should probably reconsider that approach. Eg. IME you can usually get a horse comfortable with padding(tho you might have to play around with thickness, density, etc), but as Patty said, some may just be 'too far gone' or some such, perhaps too old to worry about anything more than palliative, & in that case, you do whatever is necessary to make/keep them comfortable IMO.
.... & I just read Patty's last post above. Pretty much agree, but as said, have had no experience whatsoever, with shoeing/wedging actually improving 'navicular', except in the palliative sense, so would love to learn more about that.