you're plenty ready for naxx 10, and probably capable of doing naxx 25. As far as trash tanking, tank whatever you get aggro on. I personally don't bat an eye at taunting a target off another tank if they've got more than 2 and I have 0 -- that way we're both still getting enough rage/mana to contribute. You'll probably have less of that to contend with as a pally instead of a warr (unless you're outgeared by a DK for aoe threat).
For boss fights, obviously stick to what you're assigned, unless "it" hits the fan.
As far as the Solemn Council vs Ziggurat chest pieces... I would cringe at the drop in defense, personally, but I know the expertise is less impressive to a pally than for a warrior, as your threat skills can't generally be parried. Hit rating is harped on a good deal more for pallies, as you're dealing with spell hit values more than melee hit values. (though take that with a grain of salt, as I'm definitely not that up on pallies) I'd work on padding up the defense options you have (at least on backup gear, if not all on the gear you're wearing) because I noticed a lot of fluctuation in my defense skill as I went from heroics to naxx 10 drops. And make sure you're doing those hodir dailies every day, so you can get a shoulder enchant ASAP. May also wanna see about hunting down an enchanter to fill in the blanks on your gear. May seem a waste on blue gear you're hoping to replace soon, but it risks making you look "half-assed" trying to find groups, and you never know how long it's gonna take to get those drops...
I'm a tank. I like the beatings.