Signal a LOW level interrupt with an RTC
That page is incorrect, although it reflects what the datasheet says.
The processor can be woken from RISING/FALLING/CHANGE/LOW. I queried this with Atmel and got this response:

Commented by Manoraj Gnanadhas (Atmel)
2015-01-20 06:23:36 GMT
[Recipients: Nick Gammon]
Hello Nick,
Our design team has confirmed that “Note-3 mentioned under Table 10-1” is a datasheet bug. So you can use any type of interrupt (Rising edge/ Falling edge / Low level / Any logical change) to wake up from sleep mode. Sorry for the inconvenience caused.
Best Regards,
Manoraj Gnanadhas

This is mentioned on my page about interrupts.

Why does LOW hang the processor? The LOW interrupt is not an edge interrupt but a continuous one. Thus, while the signal is LOW the interrupt is being continually called, with only one processor instruction being executed after each call to the ISR (which itself takes around 5.125 µs). So, the ISR is not being interrupted, but effectively only one instruction of your code is being executed every 5.125 µs (probably about 1/40th of the usual speed).


and see how much of my problem is due to using an old and buggy IDE

It's nothing to do with the IDE. Change to a FALLING interrupt and your problems should go away. You may also want to clear the interrupt flag (as documented on my page) in case the interrupt event has already happened before you go to sleep. Specifically:
EIFR = bit (INTF0);  // clear flag for interrupt 0