Yes, but make sure it is dry.
In cooking Apple juice or apple cider can fill the bill.
Yes, any good quality white wine can be used instead of sherry.
Any other liqueur (e.g. vermouth, white wine, even a wine cooler) or juice (e.g. lemonade), ginger ale, or even water could be used instead (if you're dealing with a recipe). You could also omit the sherry--it all depends on what you think the result will taste best like.
You can use equal parts dry sherry/pale sherry wine; not the cooking wine... the drinking wine. :)
Not really, try sweet sherry, madeira or even sake
In a pinch you can usually make this substitution in a recipe while only slightly negatively impacting the quality of the dish. It's better than leaving the sherry out altogether. Things labeled "cooking wine" are generally the lowest quality sherry with some salt and possibly other seasonings added. So you may want to adjust the salt called for in the recipe to account for the fact that you're adding salt with the cooking wine. Real dry sherry is generally quite inexpensive and might even be cheaper ounce for ounce than supermarket cooking wine. So why not have some on hand?
In most recipes, sherry would be appropriate. Often sherry vinegar is used in place of sherry by those who want to avoid alcohol. The exception might be when preparing a salad where the greater acidity of the vinegar is important.
No, it will add too much liquid to the recipe and not enough sweetness.
Yes, but the flavors will be different. And you should use real wine or real sherry, not the salt heavy 'cooking' versions found in many grocery stores. The alcohol releases and dissolves some flavors that water based recipes will not, adding to the flavors. Essentially any alcohol should be allowed to evaporate through heating to leave the flavor of the wine used. Whether a recipe calls for cooking sherry, wine, or not, I often deglaze any meats cooked in a frying pan with red or white wine. The French say that you should use the best wine you can afford for this process but I use whatever I have to hand.
Well, its cheap sherry that you would consider not quite good enough for drinking, or any sherry that has been open to the air for long enough to alter its flavour slightly (say a few weeks) but you don't want to just throw it away.
Yes, you can use buttermilk with baking powder.
If you are looking to replace white vinegar, any vinegar can be used but it might change the taste or color of the dressing. If you are looking to eliminate vinegar, lemon or lime juice might do well.