Re: this one:
BUT …
… or am I missing something?
It’s not a ladder. White can run many many stones and eventually come back to take the stone. Black will have too many double ataris to protect.
Summary
A5 A6 C6 B6 A8
Alternatively, A5 C6 A6 B4 A4 B3 A3 and voilà White can’t go any further.
I hope I didn’t miss something this time 
What if
A5 B4?
response to 'what if'
C6 threatening to isolate the two white groups.
Possible continuation:
C6 B6 A8 A7 A4 A3 B3 A6 A4
Alternatively, if white captures after C6, Black pushes at B6.
You can try it out here
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Is seki enough?
Is seki considered alive? 
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Attempt
E5 doesn’t work because of F6, E6, F5. So only White F6 works. Next,
- If Black doesn’t respond then White can make two eyes with E5.
- If Black responds with E5 then White tenukis.
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Yes but sometimes puzzles want you to really live 
my guess
F6 E5 D5 E6 seki
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Response
After D5 black plays F5
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Rats
I think I can kill at least some stones starting with C7 cut but I can’t read out if all variations kill the whole group.
Yeah definitely you can kill half the group. Not a bad choice if you can’t find the killing move!
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If I had to guess which move is most likely the vital point I would guess B7 but I would not play it in a game unless I had to gamble the whole game on killing that whole group.