On the diplomacy side let's all feel the love a little and focus on the league and having fun.
As for a points system I think starting from the ground up is the way to go. The first thing we need to do is decide what factors should count, and a rough weighting for those factors. My initial thoughts...
Previous seasons performance
Open roster slots
I know those are the three we already have, but they're also the three that make the most sense. Essentially it should define both how the team good is and is going to be, as well as available playing time. Performance and direction could be folded into one thing too. As much as last season's record should count, I also think that the number of signed players on the roster from that team should count as well. Maybe something like this:
Overall winner - 4 pts
Runner up - 3 pts
Second round playoff team - 2 pts
playoff team - 1 pt (theses aren't cumulative, a team gets the highest one that applies)
Something based on BBM scores, totaling somewhere 10-15 points. A team only gets the points if the player is signed. Every signed player adds points to the total. Likely only the top 75 or 50 players would count for points. These points are determined at the start of the offseason, so it's not a lot of upkeep. Call this points for your 'core'.
Open slots - if the team has an open active roster slot that the player can fill it gets +6 points (8 or fewer players), an open bench slot is worth 2 pts (9-10 players) and an IL slot is worth bupkus. That's based on this years roster make up of course.
When making a bid a GM would just tack on the total points for easy comparison. It generally favours teams with less (up to a point) and better players to start the offseason, but as rosters fill the bonus for available PT should be the one that makes the most difference. I also like that it would focus on the quality of player on a given team a little more than previous season results. HtH is a very mercurial game, and playoff wins don't necessarily match up with 'best team' as an idea. I think this would also make GMs pretty interested in resigning their own best FAs as son as possible, which would streamline the offseason a lot.
Obviously the weighting could be changed to suit the majority of the leagues likes and dislikes. As far as ease of use goes, the points for your remaining players and playoff appearance is calculated at the beginning of the offseason, and all any one gm would have to do is add on the points for open slots, and add the player point totals together as they sign more players. It's certainly the case that using a system like this that a team would know before hand when they can win a FA just by matching, but I'm ok with that.