Thoughts on attrition and negotiating:
Whether or not attrition affects negotiations isn't really relevant. What IS relevant is the ability of fighters and negotiators to contribute advancement points.
Just making up numbers here for comparison purposes. If the average fighter can contribute 40 advancement points before attrition makes further fighting almost impossible, and the average super-powered fighter can contribute 80 advancement points, then the same needs to apply to negotiators - the average negotiator should be able to contribute 40 advancement points while the average super-powered negotiator should be able to contribute 80 advancement points.
For the purposes of determining the ability to generate advancement points, do not take into consideration current stocks of goods in inventory - those will eventually become depleted. And once those stocks are gone, the only thing that matters is the ability to generate supplies on a daily basis. For the average player, this probably doesn't amount to more than 100 - 200 goods a day. However, for a super-powered player, this could easily approach 5000 goods a day.
In terms of advancement points, if the average super-powered player can contribute twice as much as the average player, that still allows the average player to be relevant. On the other hand, if the average super-powered player is capable of contributing 10 times as many advancement points as the average player, then GBG will be the domain of the super-powered players.
In GE negotiations require a tavern boost. Even with the boost, success is not assured for the more difficult negotiations. This is not a paradigm that should be used for BGB.
Why not? First of all, all negotiations in BGB should have the ability to end in success (without spending diamonds). Second, the ability to generate tavern silver is limited even with the best of friends. If tavern silver is required for BGB, then the ability to negotiate for the average player (or even the super-powered player) is severely restricted. In other words, negotiations should allow unlimited tries (each try is still using up goods so there is no reason to have negotiations fail).