I have access to all girls and currently have 15 million total speed, and only spent that initial 35$.
So haw far are you in event?

The skills also don't "almost double the requirements every level", they only have major increases every 8-9 levels.
As I can see for me, level 32 required 1 day, level 33 required 3 days, level 34 requires 7 days, not sure how much will be required later, maybe it will slow down.

It also isn't the first event, it's the second, they even made participation more accessible rather than less.
It is the first event with such model. Where you have to fight fixed stat enemies, not random people. What do you mean by participation being more accessible - I got 7 rewards in 1st event, I got 4 and maybe will get 5th one in current one. So if participation is reward then it is less accessible, not more. I do like that they allowed people to get equipment for higher tier girls that are not unlocked yet.

We also know that whales did finish the event already, after all, the event gives fame and there's multiple people with the same amount of fame as ashes, who shared all the outfits and is known to have completed it. Each of those people completed the event. We could already see a whale with sub-600k stats on his dipfroat on the leaderboards at the time Thanatos posted his message, with the same amount of fame as Ashes and thus we could know that he completed it.
Sorry, I don't follow logic here. How do we know that Ashes completed the event? If it was discussed on discord don't assume that it is a common knowledge as not everyone is there. How a whale with sub-600k stats on dipfroat (the girl that can be bought for gems) show us anything about event? Can she participate in event and fight herself or is she added as a normal girl, and then I don't see how she is relevant to event. I do get the idea about looking at fame. Each challenge rewarded 500 fame at max level, so those that are at the top did complete same amount of levels in event. So if Ashes completed it, then everyone else with same score did too. But still don't see where do we know that he did complete it. So "we" don't know that whales finished the event already, this means it is not common knowledge.

Win chance (base) is estimated at 1 + (your stamina / 1000 -1) - (enemy stamina - your stamina) / 500 = chance (%), and you can't go below this chance or 1% no matter what the other stats are.
Where did you get this formula?

There has been proof of it not being the case for as long as the first message complaining about it came out. The assumption was stupid because he could already see for himself that it was not the case. You keep saying there is no proof of it not being the case, but the proof is visible to anyone who bothers looking and it's been there since before DShanks even posted.
There was no accessible proof. All proof that you provided required some hidden knowledge that is not visible if you follow this thread and certainly not if you look at one post and have an experience of ftp player. You can't estimate how much $ you need to invest before you start investing or someone shares his values. Now you provided that value and we know it ($35 to get unlock all girls except special event girl). You can't know that estimate was made based on first few girls (you needed to access separate medium - discord to do that). And even if theory is wrong right away, for example someone can say now "each level tripples Santa's stats" and we already know that it is an incorrect theory, then someone else saying "then last level will require 30B" would not make his calculations incorrect or stupid.

Anyway if a lot of people (I see 12 with 4k fame on leaderboards) completed the event, I am sure there would be some info about required stats for final level already. Why is not it around yet? Can anyone care to share?