The function is 2x/(1+x). And 2400% is enough, the final chance is 96%. You can get details here https://mazebert.com/forum/news/new-loot-function--id994/
Yes, she is still working. It means creep drop twice. One from the tower, one from the widow. That's why widow is a unique card. -- Edit:Aboves maybe misunderstanding, I read Andy's explain on discord, and I suppose that there is "item chance of the creep" and "item chance of the tower". they are multiplied to get the final droprate. Now, Widow increase "item chance of the creep", Potions increase "item chance of the tower". Widow is the only way to increase "item chance of the creep". That's the real reason why she is a unique card.
From what I can tell, assuming no reworks since that post, is that you were right up until you said you can get two items with widow. I don't know think you are right about the creep's base item chance being affected.