What it looks like, from the outside: IWM licence rights to product minis for BT from, probably, Topps (I could be wrong here.) If so, as long as Topps gets their annual royalties check, they don't get involved one little bit.
In the past, and still now, IWM commissions the production of mini masters independantly. They have to be based on CGL-owned art (eg. not on MWDA minis, unless they have CGL art.) IWM pays the sculptor directly. This is how fan funding works, except IWM solicit for the money to pay the sculptor, and provide bennies to the contributors. If it's not fan-funded, then IWM feel they'll make a profit on the sales of that mini, otherwise they wouldn't do it.
But IWM of course talks to CGL. And these days, CGL makes (commissions) plastic minis. if CGL says "We'd like you to let us get mini X out in plastic", then IWM wisely work with CGL. And if CGL says "no worries", then no problem. They may also get access to the 3D modelling done for the KS minis, which would make life easier when it came to making masters.
Given IWM is spiritually a child of FASA, and given CGL is spiritually a child of FASA, it's not like they don't know each other. But it's worth noting BT is now how IWM makes salary - they do much more work for other gaming companies. But they still loyally support us mini addicts.
The above is purely personal interpretation, based on things like participation in fan-funding, but no sekret inzider knowledge.
