Does Congress have the authority to require the U.S. Attorney General to actually be an attorney?
Congress has the de facto authority to impose such a requirement in any individual instance via their power of confirmation, I suppose.
Such an AG might be limited in the performance of his or her duties, however. Then again, since AG is an Executive position, and the AG is not someone who actually argues cases in court, that's probably not particularly important.
Similarly, the US Senate clearly has the ability to require that the Secretary of Defense be a civilian for a certain number of years as well, of course, via its Constitutionally granted confirmation power. But a law arbitrarily imposing such a power would still be unconstitutional, because it would infringe on the plenary power of the Chief Executive to nominate the person of his or her choice.
Therefore, all this talk of the necessity of a Congressional waiver is a tempest in a teapot, and, as I mentioned, I'd love to see President Trump prove that by taking it to the Supreme Court rather than meekly requesting a waiver under a law that is clearly not Constitutional...