1. It is always point and not points, regardless of how many 'things' were done.
2) Look here. I can think of a few odd contexts where you might want to say 'Look at here' but I don't think they are very natural. As an idiom, it is always 'Look here!'
2. Funnily enough it depends on the type of chair. If it is a hard sort of chair and you stay very much on top of it, you say on (i.e. wooden chair, dining chair, office chair). If it is a soft chair you 'sink into' comfortably, we say in. (i.e. armchair). We also say 'on' a sofa though, even a soft squashy one. confusing how I think about it. In fact, I think we are sitting on various sorts of chairs apart from armchair and dentist's chair, which both take in (even though a dentist's chair is hard). I give up. There isn't a rule. It is just a case of learning which is appropriate for each type of chair. I apologise for the awkwardness of my native language!