Eh, I'll help you with your questions, why not.

1. Yes, when you light up the enemy, (whether with a scanner or by pointing at them with your gun) they appear on the minimap for your entire team.

2. Unfortunately no, there is currently no direct way to get war points for using the active scanner. You could say it indirectly helps your team get warpoints by giving them information, but you don't get anything from it. CCP is aware that this really makes the scanner less appealing, and apparently plan to add ways for it to grant WP.

3. The scanners are not perfect, and can only pick things up with a high enough energy signature. So for instance a tank will always get picked up by the scanner if it's in range because it's very high profile. A scout suit, on the other hand, is very low profile and therefore resistant to being scanned. When you get the error message, that means you hit someone with too low of a profile and so they will not be lit up on your map. To prevent this from happening, put more points into scanners and buy the higher tech ones which have better scanning precision.