In DIY field you are expected to hit if not one, then another part's cost. The Cornet 2 has only basic means of protecting against possible failures. Then for:
C203, 208 have voltages of 150V and 155V respectively
They may have as much as full B+ in case of heating voltage failure, tubes failure and many other factors.
...they are tested at twice the voltage (mil-spec) and even with surge it still won't hit the 200V on the line. So would this be a viable solution?
IMHO, no. The testing uses new parts in controlled equipment and environment and for limited period of time. You have in the device neither of the provisions .
...someone ... suggested using a .033uF 400V platinum cap in C200, 206 as the bypass cap. ...that is a far cry from the 1uF required. ...why or why not be a viable option for educational purposes.
I don't think the change is suitable for the caps. The caps are used for "shorting" power supply for the signal, so they are essential and important part of the signal part. Their values have been chosen in such way that they have reasonable small impedance for the frequencies which is not covered by electrolytic caps. Lessening the values leds to changing the tone balance in the frequencies mentioned.
