illluck I'm probably missing something obvious, but does "a" actually matter?
2 seems to be only possible defense because all others give either dead shape in corner or connection, but then white can descend at 3 and get eye vs. no eye. Black seems behind by a liberty even with a?
illluck Ah, right! So I guess with the liberty at a it's a ko for seki?
Just a side note, 2 in first diagram should be at 5 (otherwise 3 at 5 will kill). 8 should be at b, otherwise there's a ko even with liberty at a.