Mark,
The H bridge serves two purposes, to convert from logic levels (5V) to solenoid levels (12/24V) and to enable higher currents to be supplied. The H bridge could in fact be replaced by a relay that was configured for changeover. You could drive a 1.5V clock direct from the pic as long as the current requirement is no greater than 20mA.
The watch crystal is to run the pic. The reason that clock crystals are either 32,768Hz or 4,194,304Hz is because these are nice round numbers in binary and so easy to divide down in hardware.
32768 = 1,000,000,000,000,000 binary = 2^15
4194304 = 10,000,000,000,000,000,000,000 binary = 2^22
Either would be suitable for your project. The reason I chose 32768 was because the internal timers in the pic are 16 bit which will accommodate 2^15 but not 2^22. Also, slower running circuits use less power and are more stable.
One reason I suggested changing to a 16F88 was because it has an internal clock that can run at 32768Hz (the 628 runs at 4MHz but could still be used.). Unfortunately, these internal clocks are only accurate to 1%. In ppm terms that’s 10,000ppm compared to a crystals 20ppm. However, because we have a signal that arrives every 30 seconds, we can use it to calculate the true speed of the internal clock and compensate in software. This would mean that the first 30 second period would have seconds that could vary by ± 10mS, but subsequent seconds would be accurate.
Where about in Aus is your Aunt moving to? I only moved here 4 years ago from Manchester. Needless to say, I’m not missing the weather. I'm also rather surprised at how bad Maplins has become! It now looks more like Tandy.
Mike.