[geezer#99]

Cam walks because of it's design. The lobes aren't flat, they're machined on a slight angle so the lifter's will rotate. The angle combines with the lifter rotation causes the cam to walk rearward. The back of the cam gear limits it. It won't show up at idle but might walk forward at higher rpm. A cam button in the cover stops that forward motion.
Low speed detonation is pinging but at a much lower rpm (like idle) than you'd normally hear at a higher rpm under a load. It's sounds like a dull rattle like a rock in a metal can.
I've had motors that had low idle detonation but didn't ping at higher rpm. It's caused by too much initial timing combined with too much vac advance at idle. You likely only have manifold vacuum available for your dizzy vac pot.
I like your mechanics statement that the slack in the timing chain causes the rattle. There shouldn't be any slack. He just rebuilt it. It should be new.