I thought you had specified that the receiving player gets to clear the area (and do whatever quests / re-runs are there). Is that a rule change?Doesn't matter. Both players can clear the area, so if the first doesn't kill the Countess, their partner can. Or, if you prefer, you can skip her entirely.
The Rule of Twos does not apply to your merc.
Rule change on the mercs?Yes, the rule of twos would apply to mercs as well.
Because of many concerns people have had about time and the amount of switching, the progress required is now an act every TWO weeks. That should give much more flexibility.