To answer the first question - I think all tanking classes are needed for whatever benefits they can provide. It sucks when you have to switch out to dps from tank, but I would also say that you can get the other warrior (and druid) to learn off-specs as well so that they can swap out when needed. No sense in being unfair to one or the other.
As I recall (from looking at my armory occasionally when I am in fury) your stance sometimes (or all the time, I'm not sure) does not factor into your numbers on the armory... so your crit is lower than when you are in 'zerker stance.
I can't check the armory right now (at work) but will have a look later to comment on gear specifics.
Depending on how many paladins are in the raid, you may be better off with your battle shout. Usually I have found with 1 palli, I will request kings over might, then give myself the AP buff. It is the same as might, it just has a 6 min uptime as opposed to the much longer might (10m?) uptime.
As for the Arms v. Fury: How many druids are in the group usually? If there are a whole bunch of them them arms may be better for the raid, but if there is only one or two then the Fury dps may actually be better. I would say if you are doing more damage in Fury, stick with it to see how it goes, and make a change if it seems like that would be better based on your average group make-up.
TBH, I think Fury has a lot of advantages over arms in terms of dps because there is little (if any) ramp-up time to top your dps, which will generally equate to more damage and thus a greater help to the raid. This is, however, affected greatly by raid content... as some fights have a lot of movement on the part of melee, whereas other fights have very little melee movement.