[Arran]

Philco tone control is more robust though requires rebuilding when being restored (usually a potted pocket with caps routed to switch) whereas Zenith tone control is a regular packet rotary switch with caps explicitly soldered to it.
This is much simpler to tackle but the switch is one wafer, brittle and requires extreme being careful when pulling the chassis out as the switch is the topmost point of the chassis and catches with the top of the back of the cabinet when the chassis is being pulled. Don't ask me how I found out and how I remedied it but I usually warn everybody, as even experienced people often overlook this - as I found out, I am not the only one.

As for the C17 and the "unusual arrangement", if you look at the Philco 66 sch, for example, you will find exact same cap (part of the cap 34, tone control) of exactly the same value in the exact same place permanently wired across P-K of the 42 output pentode. I saw similar arrangement in other radios too.

http://www.nostalgiaair.org/PagesByModel...013827.pdf

No;
  The unusual arrangement is having a tone compensation cap between the plate circuit and ground on the 6F5 first audio tube, the one between plate and cathode on the second audio tube is quite normal, and it's a better arrangement then RCA used where they would connect the tone compensation cap between the two outer legs of the output transformer primary where you need a 1kv cap to prevent failure.
Regards
Arran