I would not bother with your modifications. 1. If you add your capacitor to the base if any of the out puts are accidently shorted to earth, the capacitor will discharge through the base of Q1, which may blow it up. 2. there is already a 10mF cap on the collector, so adding your extra 1mF will only make a small difference. R2 does limit the input voltage to the 7812 when its taking current, again opening its output line or stopping the oscillator could cause it to take a small currrent, leading to excessive voltage on its input pin. It could do with a zener diode at this point to limit the input voltage.
Frank