Dark knight's inner ability

afaik yes



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



this is perfectly feasible

i can't check atm, but i think it's possible, i do remember dropping my first legendary line to the second slot (44% buff too) and keeping rerolling the top line which was legendary, i don't remember if i had double legendary prior to the patch though, and i had to lock the rank and potential of the legendary one, which dropped it and i ended up with another legendary.

either way, as it stands i don't think you lose anything by trying? if you only have one legendary line just lock it and have it drop, if the top line doesn't remain legendary then you still have your 44% buff.