The thing I have always loved about Marina's fourth wall was that it resembled a Marina newbie week--in overdrive. Characters could post, plots happened, enemies fought, wacky shenanigans occurred, and everyone is very welcoming. I love that feeling of how welcoming we are and don't want to lose it.
I voted for the visitors post option because based on the Current Characters Post option, to get 4th Wallers to tag my in-game characters I have to make a post instead of being able to tag them, and then hope they actually tag my post. I prefer being able to tag around instead of making people come to me while coming up with multiple posts about the normal, run-of-the-mill things my characters are doing that day that will be almost identical to the posts other players are making. The mingle being action only means I can only tag visitors in action-format, which is restrictive in how you can react/respond to meeting another character.
At the same time, the visitors making posts and being required to sign-up to tag can feel exclusive as not everyone will feel comfortable with having to sign up to participate in a fourth wall. It also puts a lot more work on the mods as the week goes on. Wouldn't it be easier to require visitors to sign-up if they intend to make posts, but opening up commenting to allow all visitors to tag for that week without doing so? There could be a note that any 4th Waller who didn't sign up before a certain date won't be able to make posts, but all visitors are allowed and encouraged to tag around. The Mods would not be required to keep up with comm access requests throughout the week then because the deadline would be passed and then could then close comment access at the start of the new dayweek. As the Current Comment option requires opening comment access anyway, it's a mix of the two that requires more work than option B, but not as much work as option A.
No matter what option wins, I am still very much in favor of a mingle. A way to curb the amount of posts going up, but still allow for freedom in IC communication would be not making the mingle action-only? That part seems the most unnecessary since, if we allow action/voice/video in the mingle, people wouldn't feel a need to make extra posts just to have those options and it helps the visitors who don't have characters interact easily face-to-face. Some characters are just easier to tag with over the network instead of doing in person logs.
tl;dr: I would love a mix of these two options together where visitors who sign up can make posts, but signing up isn't required to tag around, and the mingle is network and action capable to make people feel less awkward if they don't like making separate posts.