According to the info, Karambwan's never stop burning until 99 technically, now I think 85+ w/ Cooking Gauntlets it is maybe 1/250+ that you'd actually burn them from what I can tell.

Answer to first question, yes im pretty sure you stop having as many bad wines but i dont think it goes to 100%

ty mango

hmm ok ty

Straight off the Wiki, No information on exactly Cook rates / etc. So I dont think it matters what level you are, it will be the same % always.

An Unfermented wine is created when a player with a cooking level of 35 or more uses grapes with a jug of water. After 12 seconds, this item will automatically turn to either a jug of wine or a jug of bad wine, giving 200 cooking experience for the former situation and none at all for the latter.

It can ferment in your bank, but if you keep making more and depositing it in the bank, it will not ferment until you stop making Unfermented wine for 12 seconds. It should be noted that unfermented wine would not ferment if you drop it. It must stay in your inventory or your bank for the duration without a break to ferment.