It does sound to me like they're planning for a swarm, and that what they've been doing is "practice" swarms.
I had my first hive swarm last year at the begining of March. We tried to catch it, and managed to put it in a box. I think they went back to the hive though, because the next day the original hive looked the same as usual. And it didn't look smaller anymore.
Anyway... I split the hive because it looked so large.
A few thoughts -
1) They may be feeling crowded because of only having one brood box. I saw you moved the queen excluder up. That is one good move. Did you say if you have another brood box and frames? If so, you could give them that, inbetween the bottom super and the honey supers. Or consider making a split. If the hive is rather large, it would work out well to go into winter with two hives
2) Get them some ventilation. Opening things up an inch between the supers won't work. What Sherry said about bridge comb and defending the hive is exactly right. Get them on a screened bottom board (or stand), or give ventilation on the top.
My screened bottom board stand:
And lastly.... what was the brown smears on the front of the hive? I think someone on here has said stuff like that is nosema. (Robo?, Michael?, Finsky?)