What Smokepole said, in spades!
Once the receiver is destroyed, it's just a bunch of parts.
I think it boils down to process. If you want the firearm deactivated, the guidelines must be followed (note, these are guidelines, not law, but if you want the folks to sign off...), but if the receiver is destroyed, separate from the rest of the firearm, it is dealt with more directly. I would look into the "destroyed receiver" route.
Swap around, your good parts, for some worth destroying, and build up a ####'n'click from there, using, of course, due diligence to make it un-convertible.
Not sure how that would fall in respect to the "replica" rules. Tread carefully. Of course, if the receiver is destroyed, there is no clear need to destroy any of the other parts, unless you wanted to stop someone from getting "ideas".
I was involved in a couple ####'n'click conversions of AK's, while in Central America. By the time they were finished, it would have been far easier to build from scratch, rather than rebuild these, but they still cycled. Much welding and grinding was involved.
On those, we welded the chambers and bolt faces, and ground them to allow the bolt to come forward, most of the pins were welded in place inside the action (ugly stuff!) the bore of the barrel saw a fair bit of weld deposited into it, and was welded securely to the receiver. IIRC we cut out some of the internals as well, but it's been a while.
Cheers
Trev