When is Chaldea Lunchtime CE coming?

I think it’s the CE after Personal Lesson expires in the shop. If you start farming now, I’m pretty sure you’ll be able to get another 5000 by the time it gets here, especially with events giving lots of rare prisms.

You'll be fine. There's been more than enough time between shop CE's to MLB them all while clearing the shop every month, I don't imagine that will change for just this one.

if you farm mana prisms you're estimated to get like ~7200 by the time the current shop C.E. expires, so you have more than enough time to farm for the next ones

Each event gives minimum 500 prisms, so you don't have to worry about not getting them in time