beef stir-fry and fish sauce

check ingredients on bottle. Some time oyster and shrimp sauce contain no oyster or shrimp. You have garlic and scallion and sesame oil and beef flavor itself,  which I believe will dominate flavors. I would not worry

Fish sauce is valued for its flavor enhancing qualities. A fish sauce that was  fermented for a long time will have more of an intense earthy taste, than of fish. It boosts the salty and umami aspects of food.

Fish sauce will taste fine though I think that's an overly long marination for both soy and fish sauce. I'd think more about the salt impact than it tasting fishy. Fish sauce is one of those foods that doesn't particularly taste like it's name or its smell.

The fishiness in fish sauce cooks off.  Unless you are using an unusually large amount of it, your dish shouldn't taste fishy after it is cooked.  It certainly does have a flavor of its own, but is not particularly fishy after cooking.

It is a lot like using anchovies in Italian cooking, where you add one or two anchovies and mash and "dissolve" them in the sauce or dish.  Adds umami to the dish.  The Romans used a sauce similar to Asian fish sauces called garum.  It was highly valued.  I often add a bit of fish sauce to non-Asian dishes just as a flavor boost.  Works well with stews and any tomato based dish and is way more convenient that digging out some anchovies (I have a bottle sitting with the oils, soy sauces and vinegars close to the stove).  As other have noted, you do have to be aware of the overall saltiness of the dish as fish sauce adds quite a bit of salt to the dish.

like most shrimp and lobster sauce dishes(including frozen Benihana brand) contains no Lobster ?? You figure it.