Anchie, I'm so sorry to hear this. Foot pain is so complicated and troubling to those who love to walk! My DH is going through this now also with a mysteriously swollen and painful foot and no apparent reason. It's such a nuisance and worry to us both.
Your hypothesis that your foot pain results from your back pain sounds reasonable to me. It's all connected. I damaged the nail beds of two toes while hiking about 6 years ago and they have never fully recovered. A few years later I also developed plantar fasciitis on that foot. A few years after that -- unexpected knee pain on that side. I'm sure it's all related.
My nails are not perfect (and never will be) but are much better. My plantar fasciitis is cured, but the right foot still feels vaguely "different" than the left and less flexible/ comfortable in the arch most of the time. As if it could use a really deep massage! My knee problems only act up now if I run too much on hard ground. But this is after considerable physio, lots of exercise, and many adjustments with footwear.
One thing I will say -- even if the name of your foot problem is the same as someone else's that doesn't mean the cause (or cure) will be the same. Most people with plantar fasciitis, for example, are flat footed pronators. Not me. I tend to supinate and I have rather high arches. So my footwear needs are quite different from those of someone with flat feet.
During the worst of the plantar fasciitis woes, I found great relief from (adjustable) Fitflops as my everyday house and casual walking sandals during the summer. (I specify adjustable because they tend to fit rather wide and I needed to adjust the strap to a tighter setting -- if it's not adjustable it doesn't work for me.) I did NOT stop walking. I walked my usual, although if it really flared up I tried not to run.
I also invested in over the counter inserts for my shoes -- and did not have trouble fitting them. I can't wear sneakers without some sort of supportive insole -- otherwise they are too flat and cause excruciating pain after a day (even when I don't have PF). So I wear different types of Superfeet insoles with sneakers.
I'm best in shoes with a 1" to 2.5" heel or platform or flatform, and some "sponginess" in the heel area -- rubbery soles are good, and there are also special PF inserts that offer this, if that is your problem. Booties tend to work well with the inserts and I haven't had to change my shoe size. Except for the Superfeet in sneakers, my insoles are the 1/3 to 3/4 length ones.
Sandals are good if there are straps over the instep area to keep it secure -- I don't need the insert. Maybe you won't, either -- IF you do as I do, and as L'Abeille advises, and switch out your footwear often. As Donna says, it makes for complicated travel but I never regret it.
I can't wear most pumps without straps because they don't stay on and cause my toes to cramp. But I can wear pointy toes if the last is the right shape for me overall -- in fact, they tend to be more comfortable for me than rounded toes. Your mileage may vary. (Ha ha).
I would get another opinion for sure, and I'd also ice like crazy right now, and take some ibuprofen to bring down the swelling. Hang in there and don't throw any of your shoes away until you know for sure what is causing or contributing to the problem.