I can't say much about character trading, but I think I can answer question 2 a bit.
As a DK 80 who has done a couple of heroics, I'd say you will find this breed rare, especially in late game. Rumor has it that our Death and Decay generates less threat than normal.
Both my friend and I gave up as Frost Tank since we can't seem to keep the enemies' attention enough. I managed to tank until this day as Blood since I have a more decent AoE damage and Heart Strike. However, while I can save myself from low damage with self heals, I seem to take more damage than warrior and paladin even though I'm mostly def capped. Some people also says Frost Tank is better in raid.
Tl;dr : Tanking as DK will be quite a challenge, and few DKs attempted to tank, fewer succeeds
That seems up my alley! Been tankning as paladin in tbc and druid in tbc/Wotlk retade. Both are quite stright forward on how to act/play! So a challange sounds awsome!
I played DK tank during progression on retail and they were in a far better state than they are on here currently. In current raids on Lordaeron, they are bottom tier tanks together with warrior. The gap between Druid/Pala & DK/War is very noticable. Blood has decent survival on single-target bosses but has poor threat. Frost Tanks have far better threat but besides the long-duration Icebound Fortitude, they lack any real proper mitigation. The problem is that bosses hit far harder there than they did on retail and DKs simply lack the avoidance/HP pool as Druid/Block to make them not get annihilated without CDs.
I'm not saying DK is unplayable, they're just not in a very ideal state right now. Also, they are sadly limited in Heroics and such also (speed-wise) since you are so dependant on your Runes to generate threat.
Tanking in Blood is one of my favorite things here because of the absurd amounts of self-healing that you can pull off with decent gear and the fun part is, it scales to ridiculousness pretty fast :D
There are a few drawbacks, though...
AoE tanking is pretty much inexistant because of Death and Decay being bugged, like someone said in this post already.
If your gear is not on par with your highest geared DPS, they'll get threat-capped quite easily.
And considering how hard-hitting bosses are on this server and that we're very low on avoidance/HP, you're going to have some trouble early in your journey.
If you have the opportunity, I'd recommend off-tanking as Frost (great AoE threat with Howling Blast and very good for crowd control with Chillblains, Hungering Cold, etc) until you slightly overgear you current tier of content. Then you can single-target tank with Blood for lots of fun :D
But yeah, bottom line is, DK tanks are pretty ****ty, right now... :'(