Are all post-68/pre-86 imported machine guns pre-sample? If owned by a LE agency could there be any additional restrictions?

Yes.

The only way one wouldn't be is if someone imported it after GCA became effective (October 1968) for official use, then re-built and registered it with a "U.S. made" receiver prior to May 1986. Sometimes people did this to render them fully transferable. It would not be a pre-sample if that's what happened. There'd be a Form 1 or Form 2 registration to signify this action, along with the engraved markings of the domestic maker/manufacturer.

An all-original import should bear the importer's markings, and usually it's obvious to someone familiar with the exact model if the receiver is original.

No, unless something weird happened... like the agency registered it on Form 10.
Anything registered on Form 10 is only transferable to government entities. (ATF doesn't care how/if it was previously registered.)

Hopefully no one was dumb enough to break a MG's transferable or pre-sample registration status, but if they are that dumb, ATF will let them break it.

Examining the gun, the last NFA form (and possibly a FOIA request for prior forms) are the means of vetting pre-sample status.

Ok, thanks for the clarification. I’ll verify which Form it’s on.

You're welcome.

Knowing which form is a very small part of the picture. Almost any MG encountered at an agency will be on Form 5.

If you see "RESTRICTED REGISTRATION - Possession limited to continued compliance with provisions of Public Law 99-308."
near the bottom, it's definitely a post-sample. Sometimes ATF forgets to stamp them however, so the absence of that stamp doesn't prove it's a pre-sample.