So, believe or not, I finally shut my machine down and restarted for the first time since above posts tonight, so I can finally answer your last question above...
...yes, 'Option+PowerOn' showed the desired (and already set) boot disk along with my other 10.8.5 previous system partition. Selecting it (10.9.5 main system) allowed normal boot-up AFTER one successful boot, but then one restart later back to failed ('null sign') boot.
So, here is sequence:
Last time I successfully booted (which was when I last posted here), I used 'Command+R', otherwise cold boot and restart would take me to 'null sign' or boot off the other partition (10.8.5 system). I then ran machine since then without rebooting (using only 'sleep' and using 'log out' to clear any small issues).
Tonight, after a audio problem I couldn't clear up, I shut down out of necessity. When I restarted, machine correctly booted to my target 10.9.5 system partition! Feeling empowered, I took the opportunity to run an installer I had been putting off as it required a 'restart'. But, THAT restart came up with 'null' again
At that point, I then attempted to answer your question and start holding 'option', my 10.9.5 system and alternate 10.8.5 were there (and already selected) so I just pressed enter and I am back up.
So in summary, yes I can use 'Option' to choose correct and previous set start-up disk, but normal restart continues to tend towards a 'null' screen which I then have to deal with...previously using 'Command-R' to successfully boot (though no 'restore' partition exists on this disk since it is an updated original OSX 10.5 system), and tonight using 'option' to manually select correct start-up.
So, the mystery continues.