Novice Needs Help with Elderberry

I am trying to finish up about 20 gals of elderberry wine that was started by someone else. They passed away and I would like to finish this wine. I am a novice winemaker.

The person who started the wine usually didn't follow a specific recipe, so I am guessing they used past experience, judgement and probably whatever yeast they could get at the grocery store. I know they probably didn't use campden tablets. The wine is in five gal carboys. Some appears to be still be fermenting (bubbling). 3 of 4 carboys have air locks. The other has just a cork and has some white film on top.

I siphoned off some of the wine and measured the sg - it was about .9. I tasted it and it was very bitter/dry. I gradually added 1 pt (500 ml) of wine conditioner (sucrose with sorbic acid) and it tasted better. But it took the whole pint of conditioner for just one gallon. I siphoned off the balance of the wine into small containers to transport in my car. I have added additional sugar to the wine (heated a small amount of wine and let sugar dissolve into) and have returned everything to a clean carboy with airlock.

At this point the wine has a sulfur smell that I didn't noticed at first. Is this because of the conditioner? oxidation?

Will the sulfur smell likely go away with time?
The carboy with the white film on top also has some "head space" - is the film oxidation? Is that carboy beyond hope or could it be saved?

The sulfur smell is hydrogen sulfide. It is caused by a number of things, but in your case (I doubt it was too much sulfides) I bet it was from poor sanitation and the wine got infected with something, witnessed by the white film on top of one of the carboys. The way to get rid of that smell is to add copper sulfite. Since copper is poisionous in large enough amounts you can decide if it is something you want to do.

I would stay away from sweetening all that wine so extremely. First, the dry/bitter taste will decrese as it ages. If you overly sweeten a young wine (so that it tates drinkable now) in a year it will be too sweet. Finally, if you are still seeig signs of fermentation (was teh wine made THIS year?) it is WAAAAAY to early to be sweetening it. Let it age at least 6-9 months in bulk and then asses it before bottling.

The carboy with headspace - add some campden and transfer it to a clean carboy. make sure ti stays topped up and it should be ok.