Chaldea Lifesavers droprate

To answer your second question first, yes, I did.
As for the first, it depends on the difficulty of the quest, like so:
- Novice: 0.10%
- Intermediate: 0.20%
- Advanced: 0.40%
- Expert: 0.80%
- Explosion: 1.20%
- Annihilation: 1.60%

What's more, the same goes for the 5 star CE from the first part, and in fact, getting a drop is the only way to MLB that one. The second part CE can be wholly store-bought, however.

Yeah. It was possible. I got 2 drops of the event ce during farming. For me, it dropped more than the Summertime Mistress since I barely MLB it.