Is it legal for a Class 3 dealer to buy a HK Post-86 machine-gun, cut the receiver - then sell it to you as a parts kit?

Yes, assuming it is properly demilled to current ATF specifications.  I also suspect that some paperwork would need to be filed with the ATF to have it removed from the registry.

Yes, it's done all the time. And in most cases, the of he sum of the parts is greater than the whole, if you catch my drift.

A post-sample will never be taken off the registry, it will be noted as destroyed. A letter is sent to the Chief of ATF Tech. Branch (Mr. Hutchins) describing how and where (3 diag. cuts thru the receiver) were made. It's also logged it in the 'Bound Book'. A letter will come back from the Branch stating that they have logged the gun as destroyed.

I believe or at least I have heard that as part of the demilling process the serial number of the weapon has to be cut out of the receiver and used as proof of the demilling.