RESULT
So, I got the girl.
However, it was a close run thing.
I also question whether it was worthwhile.
Recall that I started with 9,600 Kobans, 5 Sandalwood, a small bank of Combativity and a World Girl already at 20.
End result: 46 Kobans left, one single charge of the Sandalwood left, a small stack of black gems, and a few tens of millions of cash.
It was a very, very close result. The world girl "ate" up a lot of the Combativity, and I eventually started using the Sandalwood even though she hadn't fallen, as I was burning through the Kobans.
I didn't use any "x10" or "x50" spins as I didn't have the Kobans for it - and the stack of Mythic Shards was in no danger of running out (it just kept growing and growing).
All of my Kobans were spent on Combativity refills. All of my savings were gone. And finally I was at 99 Mythic Shards with NO RESULT.
I got very lucky though, because the Daily Goals give you an extra Combativity, and on the VERY LAST ONE, I got a Mythic Shard. Which was, very, very lucky given the previous drop rate.
So, I got the girl.
However, this entirely depleted my Koban savings (I was hoping to be luckier). So not only did I miss out on the prior Season's girls, but am in no position to spend Kobans on anything else either. I could have easily got nothing though. I could have got many other non-Mythic girls for the same outlay.
Another key point is both how TIME CONSUMING and INCREDIBLY BORING the game play to get the girl was. "Click, click, click, refill Sandalwood" and repeat FOREVER.
I thought "Boss Bang" was a pointless clickfest, but boy, the Mythic contest so lacks any engagement. And is so time consuming. (An hour? More - I don't know - I lost track of time.)
Finally, it is a "Green Gem" girl, and that eats into other Legendary girls I have, that I may prefer more in terms of design.
She is going to be very expensive to level up, and eats into other girl's progress.
So would I do it again? As a casual FTP player?
I honestly don't know. It was (a) expensive (b) risky (c) dull (d) time-consuming (e) high opportunity cost and (f) not obviously rewarding.
Your mileage may vary.
Thanks for reading!
And a repeat big "Thank you!" to everyone who contributed.