I can confirm it's not an divider that goes down.

It's an sliding door that comes to and away from you (horizontal not veritcally)

I would suggest you take
1K and 2K. Most likely your cabin will be empty so the flight attendant would use the middle seat as an bed giving you flexibility.

When I flew as 2, we chose 1K & 2K.

1A is blocked until check-in. 1A and 2A's seat seems to be highly used compared to 1K & 2K. (wear and tear that is)