Of course they do, they cost energy! What else would it cost?
I explained this in brief up thread, if you didn't read it, do so, if you need more detail, post back after.
What energy? I can't find where you describe how it works.
What exactly do you want to know? :|
I'm a little confused.
A presents a challenge to B. B has to spend CPU time to solve it and present the solution to A. The problems are not trivial and will result in B spending CPU time solving it. These problems could be anything from a simple hash target like Bitcoins, to more exotic work challenges that consider a number of data sets.
If B has too many connections open, he will get more challenges than he can process and will not be able to provide the results in time. Should A decide to make a transaction before B has provided the solution, B will not be eligible for an endorsement.