Dolarius: for the first points please read
Read first! Messenger Plus! Live FAQ. For the second that's because of the difficulty of using groups with WLM, noone has really found a good way of doing it to my knowledge and hence Patchou has not implemented the feature. And for your third have a look at
Patchou's reply to Idea for a future version?. I hope this gives you all the information you need.