Is it the right use of the RTC RV8523 to wake up an atmega328p?
Only level interrupt can be used for waking up by INT0/1. Level change interrupt does not work.

Additionally, if there is a need to process interrupt (not just wake up), then it should be held low during entire wake-up time, which can be as long as 258 clock cycles. Otherwise the MCU will wake up but the interrupt will be lost.

UPDATE

Not directly related to the question but might be worth considering.

The 8 seconds limit on sleep delay that forced you to use external timer is pure artificial. It is direct result of the library that you are using relying on watchdog timer as wake up interrupt source. The reason they use watchdog is that WDT is the only internal interrupt source available in all sleep modes. This makes writing the library easier, but does not mean the end result is the best that could be. 

The lowest power sleep mode is Power-down. Watchdog, external interrupt and TWI match are the only possible interrupt sources available. This forces you to use external interrupt for delays longer than 8s. Total power consumption is power-down + external clock, or 0.5uA + 0.13uA = 0.63uA

Let's take a look at Power-save mode. The only difference from Power-down is that asynchronous clock is active, allowing you to use 32.768 kHz watch crystal as clock source. And it can be used for wake-up interrupt. Total power consumption is Power-save, which is Power-down + internal clock. According to datasheet the current of about 0.95uA is achievable in this mode (although you need to do some advanced software tricks for this, like disabling brown-out, shutting down ADC etc.)

The difference between these modes is two times less than even ultra-low quiescent current LDOs consume (about 0.5uA). Considering that this allows you to get rid of all those external parts I'd say it is quite viable option.

There are two drawbacks to consider. First, since the same pins are used for watch crystal as for normal crystal oscillator you are limited to internal RC oscillator as system clock source. Note that internal RC can be calibrated by user to about 1% accuracy, which should be enough for most applications.

Second, there is no button cell backup you've mentioned before. However since this is battery-powered application (I assume it is, otherwise you'd control power adapter by the clock, not the MCU sleep mode) then I see no need in backup. A battery is a battery. You can add simple mosfet switches to shut off power from the rest of circuit and to connect battery to ADC pin. Then you'd measure battery voltage on wake-up and disable everything but RTC if it is getting too low.