Does the plural “let us” (Gen 11:7) indicate that God was speaking to some other spiritual beings, or what?

This is not altogether clear, but see comments on Gen 1:1 above for some notions. In this case, a significant possibility is that God was accompanied by assistants around the time that he punished the evildoers to confuse their language, as when two angels were with him before he destroyed Sodom and Gomorrah (Gen 18-19), and when “the destroyer” (Ex 12:23) slew the firstborn of Egypt. This is the sort of detail that the Bible might well leave to implication and inference, because the fact did not merit direct statement.