I m thinking on two approaches to the problem, WCCP or patching Squid. Ultimately the complexity and time decides the way.

Patch for HTTPs filtering.
I have to remove the existing version and install a new patched version from source.

I would start with PBR in the first phase for supporting the external proxy.

Thanks both.
As suggested, is there a way to check the device is live and then forward traffic or do a fallback to another device.