What is the opposite of the Devil's Advocate?

In current common usage, "playing devil's advocate" is arguing a position you do not genuinely support. It is not necessarily arguing 'for' or 'against' a proposal - it could be either. So, I think either situation you suggest would fit. (If you genuinely support the proposal/position/idea, then you're simply agreeing, or supporting, or as Jack suggests advocating.)

If the proposal is "Let's move to a new house":

"I don't think we need a new house, but to play Devil's Advocate ... "

"I agree we should get a new house, but to play Devil's Advocate ..."

Either of these looks fine to me. "For the sake of argument" is used similarly.

(jwpat7 has a more interesting answer, but I don't think the terms/offices he suggests would be recognized in the same (idiomatic, common) contexts as D.A.)

As noted in the Devil's advocate wikipedia article, the advocates on the opposite sides of canonization arguments were called the Promoter of the Faith (Latin: promotor fidei) or (popularly) Devil's advocate, on the negative side; and on the positive side, the Promoter of the Cause, or God's advocate (Latin: advocatus Dei).

As I understand it, the promotor fidei office was abolished by Pope John Paul II in 1983, since which time the Promoter of Justice (promotor iustitiae) has been in charge of such examinations.

I think your question is about the idiomatic usage rather than the historical term, information on which jwpat7 has already provided.

I would think you're simply an advocate.

I guess that the expression (A+B), like white salt, has 3 reciprocals: you can negate A, negate B or both. You may have white sweety, dark salt or dark sweety. Not to say that white and salt have different aspect that can be negated. Do you want all 3 inverses? I think that the answer is just a right person.