Lewyn

Forseti essentially gives Desperation when he's above 50%, meaning he still has to double naturally. Giving him Desperation would essentially make his tome's effect always active, though for that only Desperation 2 is needed since his tome already goes to 50% hp.

Yes, it would stack.

Yes, he would be essentially a "brave-tome" like Reinhardt (though you will have to meet the speed necessary to double and skills like Wary Fighter will cancel him).

But why would you want to put desperation instead of the reliable special spiral? If Iattack 2 times I expect to get the kill, not receive the retaliation damage. Pair that with moonbow and you will have it ready for the first attack always.

He would still only double if his Speed meets the threshold requirements, but yes, Forseti + Desperation would allow him to follow-up before the opponent regardless of his HP.