Dark knight's inner ability

(I think there is no cap at least?)

afaik yes

if my 1st line drops in order while maintaining itself down to the 2nd slot, does the 1st line remain legend-ranked (basically I end up with 2 legendary lines)?

using honor points, iirc, no, only circs keep the rank of the line, but i can be corrected on this

if I roll the inner to find +1 attack speed, the other lines will most likely end up being epic most of the time. Is it realistic to get, say, 20-25% buff duration on the 2nd/3rd line?actually, is +1speed AND 20-25% buff more beneficial for a drk than a pure 50% buff?

this is perfectly feasible