I have a cousin who is red/green colorblind. If you show him something red, he can tell it's red. If you show him something green, he can tell it's green. If you show him the two together, though, he can't tell them apart. Since he can tell whether they're red or green when shown them separately, he knows he's missing something when he's shown them together.
I steal money from my parents all the time. I never felt bad about it. When I get caught, however, I feel terrible. I thought that was guilt until I realized I only felt bad when I got caught.
In other words, I can't tell whether I'm feeling what other people feel until I check the basic idea of the feeling (good, bad, bubbly, sick) against the situation that provokes it. Unless both match, the feeling's probably not the same. In the above example, the basic ideas of the feelings matched (guilt=bad; the feeling I'm having now=bad), but the situation was off (normal person+theft=guilt; me+theft=:); me+caught=bad feeling).
Faking emotions is quite easy as people see in you what they want and expect to see.
Can a sociopath understand the emotions you mentioned? Yes he can, on an intellectual level atleast. The same way a bling man knows what something looks like if you describe it to him in detail. The same way he knows that, for example, the statue of liberty is magnificent because he is there and people around him go "wooo" and "aaah it's so awesome". With these experiences of observation of others reactions and descriptions, he could convince another person that he has seen it, even though he has not.
I don't think they are fully blind though, otherwise they would be robots. They lack the range and depth, barely even noticing emotions.
Yes, it's quite easy to decieve yourself into thinking you are normal and the thought that you know you shouldn't do something quickly turns into thinking you feel as you shouldn't do something. Afterall, what do you have to compare the experienece to if you never truly felt guilt?
Some don't even realise it and some, like you said, know but don't care. Why would they? Does a blind man care that the sky is blue? The colour of the sky has never affected him in any way, shape or form and never will.