Replacing Leaking Compression Fittings

I had a new in-floor heat system installed 9 years ago and the fittings on these manifolds (see pics) were not installed well and have been leaking and are getting worse and worse. I want to fix this issue which I'm guessing means throwing away parts of this set-up (scaled-up and corroded) and using new parts. My questions are:

* Am I just changing out the compression fittings on the top of the manifold? Or do you think other parts will need to be replaced.
* Do these compression fittings thread into the manifold?
* Where do I buy fittings like these and what are they called?
* See the "Supply Side" picture: "A" is a bleeder valve right? "B" is a hose nipple, but there isn't a valve between that and the manifold. I don't understand. It would be nice to have a valve here for when I fill it back up with water. Or is there a one-way valve between the hose nipple and the manifold.
* Should I be cutting off the 1/2" PEX and starting on a "fresher" piece
* How much torque on the compression fitting to get no leaks?

Thanks guys!

Carl