If you buy the second event CE, you should be getting +10 (minimum) per run, either standard or rare dango (probably the latter, given the red chest rate).Well I only have the free +1 CE and only recently bought the +3 CE. I sunk about 5 ticket and 40K FP for nothing. So every run gives me +7.
1 hour = 12 AP
1 day = 288 AP = 57.6 Run of 5AP
If you buy 2000 dango CE, then on average it gives about +6 per run (assume a drop rate of 2 rare 1 normal). This means the expected rate of return is 345.6 dango per day of owning the second piece. Doesn't seems worth it when the event only has 4 days left.
Actually, at this rate I won't even have enough dango to clear the shop nevermind the 2K CE.
57.6 run x 16 dango x 4 = ~3700 dangos. All pieces, Monument and fou = 6000 dangos.
+3 for 500 Moonfest CE
+3 for 2,000 Moonfest CE
+3 for friend Moonfest CE
+1 for story quest Mooncell CE
That's without any additional Mooncell CE, from either the FP or premium gacha.
Anyway, grabbing the second CE isn't really needed, just go with the single one you have now. I get +11 with every run, since I lucked into few additional Mooncell CE, but this won't be everyone. Just grind with the +7 you have now, and hope you get another Moonfest CE as a drop. If you don't, and need to make a sacrifice, I'd maybe pass on the +HP Fou, as the +ATK ones are more valuable than +HP. As for the Pieces/Monuments, you could probably pass on the Pieces too, since they don't seem too terribly difficult to grind for, but the Monuments you definitely want to grab all of them, even if you don't have a class right now that uses them. You will eventually, and will be happy you have them in-stock.