The reason why it's not joseki is that white's original pincering stone is not under much pressure, but it's blocking black exactly where black would want to extend to secure eyespace. If you look at the 5-4 joseki where black attaches as in your diagram, black follows it up with an extension down the left side. White's pincer stone here blocks that, so black has to move into the center, but he doesn't have clear eyespace, he's not exerting a lot of pressure on the white upperside group, and the left side stone is light and flexible.
Regarding your
I think that black's shape is a bit too weak for this. White should be able to peep and cut or something similar, without reading into it too much.
)
pincer around grey regions
with 
, but does not pull back at (a), and pincers instead.
, then B blocks at 
.
.
tenuki