I'll try to answer all of your questions in order, in response to your first question I don't know about marvel select figures in general because I don't collect many of them but at least in the case of the marvel select venom it is a simple ball joint in the torso you can see picture of it dissembled in jin saotome's tutorial for customizing the torso here- http://jinsdangerousblog.blogspot.com/
to answer your second question I have at times sprayed a figure with krylon before I started painting it to help the paint bond but in general I think a spray on primer would be more the the way to go.
To answer your question about spraying a coating in a small container and brushing it on, that doesn't work so well if you want something to brush on then look for brush on clear coats that come in a jar,
they sell gloss, semi gloss, and flat coatings so that you don't haft to worry about the different parts having different shines. Here is a list I made of different paints, and materials that might come in handy. viewtopic.php?f=15&t=41631