1. AFAIK, you cannot detect file transfers with scripting on its own. You will need a packet sniffer, such as Xniff that you will find here in the forum.
a. What john-t said: Plus can auto-accept activities including file transfers.
2. It can be difficult to determine in a 2 person conversation to find out which email address sent a particular message. There are code samples on the forum somewhere. I'll edit this post when i find some examples.
3. See 1. Not without a packet sniffer.
a. If you get the email address of the person trying to add you, check out
this script by deAd