Leave witnessing in GD. GD is already terrible to the point of being unreadable, no sense in moving a bunch of its junk to one of the good forums.
Maybe we need another Pit, with a twist. A “Let’s heap mockery and abuse on these crazy religious posts and the people who posted them” forum. We put the witnessing there, and pretty soon, no more witnessing.
Maybe it’s just me, but I think religion is so, er, special it deserves its own forum for the attention it deserves.
Witnesses would resist putting their threads in IMHO because they fail to recognize their statements as being opinion. (Although to be sure, the also tend not to recognize that there’s any debate to be had either.) Furthermore, their opinions often aren’t so humble.
The idea is that Great Debates is for discussion of issues that have no real resolution (such as politics and religion and so forth.) The person doing the witnessing possibly doesn’t want debate, but others might. Hence, it was thought to be a reasonable place to restrict such threads. Frankly, the bottom line idea was to keep them all in one location, so they don’t infest other forums.
Don’t forget money.
Perhaps true, but what does the audience want to do?
And then a Pit thread.
Hallelujah.
In that case you should go all in – a “witnessing” forum. Let people witness to each other, and the rest of us never even need to see the threads. And you can make moderating it a punishment for mods who piss you off. OR, you can make moderating it a short-term “prize” for us regulars, like “mod for a month”. So we can go fuck with them under the guise of authority.
Or just don"t open it.
The problem with putting them in IMHO is that then they can’t really be challenged: their is debating in IMHO, but stuff that would be called “questioning the underlying premise” in GD is “threadshitting” in IMHO. If people want to argue with religious threads, they need to stay in GD.
NIMBY. 
(ETA: That’s just a joke, by the way. I do think witnessing, if it must be posted at all, belongs in GD.)
The forum for things that can be proven with cites and facts is GQ. GD, by its nature, deals with things that cannot be conclusively proven, else they wouldn’t (or at least, shouldn’t) be debates.
I think this all depends on WHAT the debate is.
The Natsarim thread was entirely witnessing and didn’t ask us to hypothesize or to debate anything specific, so I think that should be in IMHO.
If it’s a specific debate, like, was Jesus born in Nazareth or Bethlehem, sure. But a “Hi, everyone. I’m a believer in [insert faith],” with no real debate posed, maybe IMHO for those.
It’s not that I don’t understand the OP, but changing rules will create new problems. Like this:
Agree that others will debate on you witness, or don’t post in a message board. When posting in GD, use a descriptive title, so that your thread can be skipped. Isn’t that enough?
The “I Must Share” clause of MPSIMS is certainly the closest fit for “witnessing,” though I suppose that witnessing believers would take exception to the “Mundane Pointless” clause, making that idea a nonstarter.
Edit: I know! Make a forum titled “Transcendent Important Stuff I Must Share” (TISIMS).