I have included some answers in bold.
I've read through forums so I have an idea on some things do but have a few other questions:
Is there any way to nail down its actual age? It is a patent pending 69-72 vintage, but the wheels were replaced with generic lawn/garden ones, will try the magnets to see what kind of legs I've got.
There are a few historians here than can possibly help to pin it down a little more. Hopefully they can chime in.
The legs keep falling out when moving, is there a trick to locking them in?
There are a few non-invasive ways and few invasive ways. Some people use anything ranging from foil, foil tape, dowels all the way to glue and screws --which are not generally recommended (yes I have seen a PP kettle that had a wooden dowel, glued into the leg tube, and attached with a screw to the socket).
Big fan of the slow & sear (SNS) but the distance between the coal grate and grill grate seems to be narrower, will any model of SNS fit in there?
No, you need the "low profile" SNS. You could also make a "Marty basket" or use a Snake/Fuse burn in there.
On these vintage kettles, aside from general child abuse - what causes the porcelain to chip over time? I ask because the lady who sold it to me said her dad loved and took care of it, so I doubt he was beating on it.
Heat. Expansion and contraction causes issues with the porcelain like "pops" and crazing.
I know that painting the exposed metal on the kettle typically doesn't work out, but has anyone tried the high heat enamel with ceramic paint?
I used it once to touch up a kettle. I personally wouldn't do it again and wouldn't recommend it. I'd just live with it. Color BBQ's experience this so much more than the standard black ones.
This will be my 6th Weber (97 Genesis, 03 Gold, 09 Performer, 22 WSM & WeberQ) I am so looking forward to cleaning her up and taking it for a spin this weekend!
Congrats!