I found an article (Polish) that may shed some light onto the origins of this mysterious dish. The first two paragraphs in translation:
"If you were to show the Greeks fried hake or pollock filets, covered with grated carrots, braised until soft with onions and tomato concentrate, asking if they could recognize it as a Greek dish, they'd probably nod their heads in an affirmative manner. At such time it's good to know that this gesture has the opposite meaning in Greece as compared to us.
"Most likely, the original recipe was psari plaki - fish baked in a tray, covered in tomato sauce with vegetables. The vegetables are prepared separately. Onions and garlic are sauteed with olive oil, sometimes with the addition of carrots and celery, perhaps some olives, a spoonful of honey and always - a lot of tomatoes. This is braised briefly with the addition of white wine. The sauce is delicate, fragrant with the wine, herbs and full of vegetable chunks. The fish is gutted, rolled in olive oil, salt, pepper and lemon juice and prepared for baking by covering it with the vegetable sauce. It should not by any means be fileted, as it is detrimental to the flavour."
So as was to be expected, our national variation is probably at most a bastardized version of a Greek dish. But who cares, it still tastes great. ;) I just made my first batch and it was delicious, I highly recommend it if you have the opportunity. :)
Kudos to your wife for giving it another shot!
Let me start with what I feel is the most important part of my answer: find a good fish monger and make friends. If the supermarket is the best you can find, so be it, but learn the name of the person behind the counter, ask about the fish, be interested. Ask what's just in, ask what's fresh. If the fish is frozen at sea, ask if you can get some from the freezer instead of the stock in the case that's been thawed and sitting all day.
Ask to touch and smell the fish. Look at the stock - do the whole fish have clear eyes? Does it smell like a dumpster? Getting the freshest, best fish will help you and your wife enjoy it more. Some good suggestions here on shopping for fish.
Now, on to your bullet points. Fish should never smell or taste "fishy". Fishy smelling fish is a sign of bad fish.
I find that a lot of my friends who say they don't like fish have had experiences with poorly handled or old fish. Fish must be fresh (or frozen at sea and handled properly afterward) to be good. I've stopped ordering fish in restaurants unless it specializes in fish (and I don't mean Red Lobster) or is well known for their freshness.
That said, some fish is certainly more strongly flavored than others. I found this nice chart here with a few varieties of mildly flavored fish. I do agree that flaky, white-fleshed fish is generally the mildest. Flatfish, like sole or flounder tend to be consistently mild. Tilapia has become an extremely popular fish lately.
However if you do find a good fish monger, talk to him about it and you should be able to find plenty of interesting venues: monkfish has been called the poor man's lobster, and I had some arctic char the other night that knocked my socks off.
As for why white fish tends to be less strongly flavored than meatier fishes, I can't really answer authoritatively. My guess would be that meatier fishes tend to be more oily and fatty, which equals more/stronger flavor.
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James Peterson, in his book Fish & Shellfish, recommends char, blackfish, grouper, ocean perch, rockfish, small salmon, trout, steelhead, and striped bass, but not pollock. However, he does say "almost any round fish can be prepared this way", and pollock is a round fish.
I suspect that the reasons that you don't see pollock suggested for cooking this way, or for that matter any way other than fried, is because the fish has a reputation for being delicate, but also slightly fishy, and drying out easily. So the risk of cooking it in a salt crust is that it could end up both dried out and extra-fishy tasting. On the other hand, salt-dried cod is its own staple ingredient in a lot of cuisines, and pollock is closely related.
In my personal experience, freshness and quality count for a lot, regardless of fish variety. If I had a couple really good, fresh, whole pollock, I'd probably try it, particularly if they are Alaska instead of Atlantic pollock.