Thanks for sharing the story. it must have been awful not knowing if you would get it back at all. Evil forces are constantly trying to separate us retro-tv folks from our stuff.
Recently, I traded a last-GE roundie for one of these last of Zenith 21" round color sets too, in an Early American Maple cabinet. Not the most desirable furniture but clean. After all, the insides are what sold most Zeniths.
Once I get mine from safely buried in the garage into the house and up to a third floor room, Ill have a look at the standard Zenith issues. Bad caps and faulty controls, plus the horiz eff coil which flies apart may have been changed from earlier chassis, Im not sure.
The CRT socket is not a big deal. I replaced a busted one on an RCA of mine here.
http://www.videokarma.org/showthread.php?t=267995 the outcome was good using the flowable silicone. Use a vacuum desoldering tool and heat each pin to remove solder, bend pin a bit and it comes right off, but do not to move any of the 12 copper-plated steel wires right where they emerge from the CRT. If you need a good base, a plug-in CRT brightener is one source.
A Zenith may be more adaptable to S-video than to composite video, the sync in may need to be reduced in amplitude and possibly inverted before feeding to the 6BA11.