The bosses are always going to get stacks, and they fall off as soon as the bosses are out of range of each other. So long as the Tanks are topped off by the time of the switch, and the Zorn tank doesn't get any reflected damage from Fearful, there'll be no problem. Also, the Zorn tank is ALWAYS going to get Fearful, which is the whole point of the swap in the first place.
You describe the mechanic correctly, but are unable to understand the tactic I wrote on the first page, so let me explain to you. Being topped off is naturally the best plan, for every situation, however in Nightmare that is hard as the amount of damage is very high, it is possible but at the same time hard. Regardless, when Toth jumps, they start to get stacks and they will continue getting them, then the tank that hard Zorn taunts Toth, and when he starts to get back, he loses his stacks before he hits the tank. Same on the tank whom now has swapped to Zorn, his boss, when it comes in range of the tank, will have lost his stacks before he can hit the tank.
Fearfull is a problem if the groups DPS is "to high", as the switch happened at specific % at the bosses, one tank may still have the fearfull debuff from the previous jump. However using the tactic written above (more details on first page in my post), you are able to delay the encounter of Tank vs Boss until the tank has lost his fearfull debuff.
It is correct that the entire swap centers around the fearfull debuff, you are once again able to describe a mechanic correctly but not able to understand the tactic that I wrote on the first page. Using the tactic I wrote, you are however able to handle a situation when BOTH tanks have the fearfull debuff (one tank still have 5-10 s left on his debuff).
Tank Zorn at the tree over by the Cave entrance and tank Toth just under where the Task Master leaps down. Never have to worry about fearful.
Where you tank the bosses are different from most guilds, however how my guild and MoX, Chosen, Ace, FriendlyFire tanks it (all guilds 16man EC Nightmare) is considered one of the best ways, not the best, there might be other ways, but it is one of the best. Feel free to try it out.