I would talk to horse owners in the area. Look at their horses. If you like what you see, ask about their farrier. Check with the local vet.
Should you expect a shoeing farrier to be able to give a good barefoot trim consistently and vice versa,or are there *specialists* to look for?
I believe a good farrier is knowagable enough to do both properly. Unfortunately you can get bad trims or shoeing from either, due to lack off knowledge of anatomy. In my opinion there no farriers or vets that can just eyeball hoof balance.
Is having an owner using a rasp in between trims to round off and keep toes from chipping preferable by farriers, or does that annoy them? If not, what kind of rasps would you recommend (medium coarse etc.)?"
I believe if your farrier is getting you to rasp between trims, he is getting you to do his work. The only reason for this would be going too long between trims. If yours is consistently asking you to do this then maybe he should only charge half price, because if you are doing the work, what is left for your farrier except to collect the money! I have heard other farriers talk about this and it come down to a fast buck for them.
Is shoeing always necessary where there's loose rock mixed with ground, or are certain hooves (color, supplements etc) able to handle it through careful owner-observation?
No, if your horse has good feet and you are not riding too hard thay should be OK. Colour has no effect. I have seen bad feet in every colour. Some supplements can promote healthy feet.
In a horse diagnosed with cushings and other senior citizen moments, is it best to have more frequent trims to prevent sore feet before it happens? (besides limiting sugars; new grass, grain etc.)"
This depends on what is causing soreness. Example: if his feet are sore from being a sinker. He would already have soft soles. So you have to be very carful to not take too much off. If it's from excessive growth then trim more often.
How did the original injury become what it did in those photos? Is that excess hoof growth trying to self-heal? Did the injury cause lameness, too? in relation to this post about GW
Young horse with sore heels.
The first thing to do is balance him medial lateral using a T square.Then shoe to 1 1\4 break over . Bring his heels back to alignment with center of cannon bone. Check diagram in under run heels. This problem is often farrier caused from trimming to much heel off. The one thing we keep forgetting is to balance the bone structure. We read about putting frog on ground so we just cut off heels with no thought to it. We must always remember to balance the bone structure!!
This horse was just a little off with slightly under run heels and out of balance medial lateral.
The owner said she couldn't believe the difference.