2. How do you mean? If you had 0 invites they were increased, if you had more than 1 than yes they were reduced, or do you mean the monthly allotments? As for the monthly, this is a security decision, it's great that some people are the perfect inviter, but a lot of people aren't, and things like this will reduce the risk of invites going out in shady ways, where there is no more vetting of the new user than "show me proofs" that can be easily faked. I personally don't care about traders/sellers except for the fact that the way they grant access to this site allows for a very easy pathway for mpaa and other dangerous entities. Not to say that having invites at all won't allow a pathway, but reigning in the availability will make it more difficult.
3. Security homey, we feel that this move is the best we can do to compromise between the security and longevity of the site, and the availability for new users to join. We did pretty much just fine with closed invites, so there isn't a huge need to increase users.