I really think it is an event. I think they may be cautious about the timing till they see what is a good length. With some girls taking 2 weeks to coach, I could see many players hoping this event would stretch for a significant length of time.
I'm curious what the countdown you saw said. Was it a month? I agree with you on it having the feel of a new mechanic, but it is also unclear which parts are related to the event, (if I can assume that at least part of this is an event), and which parts, if any, are part of a new game mechanic.
I think the diamonds have a purpose in the game. I'm guessing they expect to see players spending them on energy to be able to coach more girls. If they can get players used to the idea of spending them, by making it a bit easier to get them, they have changed the player mindset in a way that benefits them. I don't think there is anything wrong with this. It looks like it could be a win-win for game and the players.
I hadn't noticed this! I changed the wiki page for the event.
I managed to get the twins past Indira by not practicing with Indira. One of her stats is still higher, but not the sum. So, it's only the sum of the base stats.
I have girls now that have all been coached, but have both the sum of their base stats and each individual stat lower than my best girl. I still can't coach them, so there much be a threshold to pass before coaching is an option.