Need a non alcoholic substitute for white wine in pasta or fish dishes

If if it were a small amount, I would probably use a tiny bit of a sweet vinegar, like Muscat Vinegar.

I'd highly recommend you find or procure via mail order Verjus. We use it here all the time in Australia. I had never heard of Verjus as an American till I moved here. It's grapss that have been pressed unripe for their juice. The original mustard in france was made with Verjus. It can act as a substitute for wine without the alcohol. It is not as sharp as vinegar by even a measure but adds a lovely back note. You can also use it to make a sauce with butter, etc. It's a great substitute for wine in any dish.

This is probably too late, but how about verjus?

Chicken or fish stock are very good for deglazing.

I would try lemon juice or a good vinegar--start with a small amount and add more according to taste.

Better grocery stores stock de-alcohol wine. Lucky you, the white is much better than the reds.
You can also bring you favorite white wine to a boil and cook-off the alcohol, although some say some alcohol remains. Regulations allow 1% in most areas as the max allowed to say the beverage is alcohol free.
At the end of the day you want the flavor and no alcohol,so, if you are serving wine to guests I would boil off the wine you are serving to use; otherwise try the grocery store option.
Good food makes good company, wine is secondary.

Chicken or fish stock could be a good substitute, depending on the recipe (definitely for the pasta). You may also be able to just skip it in some recipes. Maybe a spritz of lemon juice for the fish in parchment?