6) Well, from a usability standpoint, I think that the best answer is A > B > D > C. Why?
Really, liquidity mining is stupid. It's similar to transmining, creating effectively negative fees to create the impression of activity. People should use a product if it's useful!
True, but the scale here matters. FTX has $1b/day and pays about $750kmonth for liquidity; Balancer has roughly no trade volume and is paying $12m/month for it.
In particular:
if payments are in line with volume and use, they help get more liquidity.
If payments are way greater, they create unused liquidity aimed at the incentives rather than trading.