1. it is active all the time
2. Yes but the correlations are unclear, a german U21 center one times raised like crazy in Rebounding after beeing an ID monster. Also Kienbink, improved pretty fast in OD after get trained in one on one mostly. But there are only rare cases, it is so obvious, and now even all skill have a slight impact on training speed.
3. In the past there was the theory to train one on one first, because it is secondary of all guard skill and most likely connected. If it works no idea, but i wold choose the skills like that to gain an increase in training speed.