So is the current theory that Melisandre had the kid burned to resurrect Jon all along or what?
I definitely don't believe that for a second. I think she genuinely thought that would help Stannis and was what she was supposed to do for the lord of light. In a round about way, though, I think that was what she was supposed to do for him, she just doesn't know it yet. It melted the snow and allowed Stannis to be defeated and her to be so distraught she went up north to where she really needed to be.... where Jon Snow was.
Considering her disposition when she left and when she got back, she didn't expect it to end up that way. She thought the snow would melt and there would be a glorious battle and he'd win. She knew everything was crumbling for him, though, and that caused a bit of crisis of faith, most likely.
Remember the lines that Thoros said earlier in the series. He talked about how he was down on everything and had nothing when Beric died and he leaned over him and said the words and didn't expect anything. Something like that scenario is happening with Melisandre.