The Brooks saddle is great although it is pitched forward... it should be fairly close to level for comfort's sake.

The valve stem can go anywhere it wants in relation to the tyre... I place them where the psi markings go so when you go to fill that tyre it's easy to know what the tyre is rated at.

If it's fixed, it should have some retention although I know quite a few folks that do just fine without it.

I suspect the frame is too small for you... that is a lot of seat post.