#1 : Did you test any acid levels of the must when you started? The titration kits are cheap and pretty easy to use...I forget which acid/s they test for (tartaric I think?) specifically.
#2 : Without knowing what acids are at what levels, it is hard to say what you should do. Reducing acid levels (such as by adding sodium bicarbonate) will affect flavor, so don't do anything to the whole thing without trying it proportionally in a small sample first. Personally, I will up the acid levels in the beginning if my titration test shows that they're low. So far with various juice and fruit wines, I've never had one yet be too high in the beginning that required any sort of reduction that dilution wouldn't fix.
#3 : Sweetness can be done after wine is stabilized, usually after fermentation is done. If fermentation is still occurring, you could attempt to arrest it with campden tablets, but in my experience this is easier said that done. YMMV. Once the wine is stabilized with appropriate amounts of crushed+dissolved campden tablets (and gravity is constant for several weeks), add a preservative such as potassium sorbate, and sweeten with sugar to taste. Let the wine hang out for a few more weeks and re-verify that your gravity hasn't changed. If the wine is held steady after all these weeks, you can go ahead and age or bottle as normal. Make sure that the gravity is super steady before bottling, or else they'll blow their corks and make everybody sad.
...Or, use non fermentable sweeteners (splenda, Equal...etc.) and just sweeten to taste and bottle after gravity has been constant for several weeks, following the end of fermentation and secondary/tertiary clearing. Personally, I find the taste of sweetener better in coffee than in wine where it is more obvious.