Inheritance to next of kin, legal.
Inheritance to ANYONE named in a will is perfectly legal.
To your second point - Also yes. The upper would either have to go on a pistol configuration in Maryland or be sold to someone out of state.
The upper could also be sold to a Maryland Resident, there is nothing in state law that prohibits it being bought, sold or transferred to a Maryland resident.
The lower would then be restricted to an HBAR upper and only an HBAR (assuming lower stays in MD). It cannot be made into a pistol. One could register it as an SBR though, but any upper attached must have a barrel under 16 inches.
But for me, if I had any preban lowers I would treat them like heirlooms! They'd never EVER leave my personal collection.