Can I render an instructional video for a commercial product in UE4 with the Control Rig Mannequin?

Yes.
That would fall under Non-Engine Product per the ula.
ULA

Section 1, A) a.
(also likely A) d. )
— edit
No on royalties, unless you’re really cleaning up on this operation.
They don’t require royalties on the first $1 million US now. I think you’re safe on a demonstrational video.