Watchdog timer for powering down until period of time elapsed

I’m using the watchdog timer to power down a couple of arduino nano and want to set a different period than the ones given by the prescaler. For example I want to set it to 1.9 seconds.
Using the code below I can set it to different periods (16 ms, 32 ms, 64 ms … 8s).

void delayWDT(byte timer) {
  sleep_enable();
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  ADCSRA &= ~(1 << ADEN);
  WDTCSR |= 0b00011000;
  WDTCSR =  0b01000000 | timer;
  wdt_reset();
  sleep_cpu();
  sleep_disable();
  ADCSRA |= (1 << ADEN);
}


ISR (WDT_vect)
{
  wdt_disable();
  MCUSR = 0;

}

If I write this in the loop function:

delayWDT(WDTO_1S); 
delayWDT(WDTO_500MS);

I presume it will power down for 1 second and then power up for a brief period and power down again for 0.5 s.

Can I make it power down for a specific period without it powering up briefly like above ?

Thanks.

No. The Hardware will do what it was designed to do.

Ok.

Could I add an external crystal and use it as a timer (with better precision compared to watchdog timer) even in power down mode ? Would a 32 kHz crystal suffice? or 12/16 MHz? would they use too much energy?

Thanks.