Pretty sure if this is made into a guideline, the only exhaustive explanation you'll see is people arguing that them finding mods and bns is enough contribution to warrant them only mapping 1 out of 3 or even 4 diffs, and at that point the guideline would basically be useless since that's what most mappers do regardless. Once a few maps get through like that, even bns and qat wouldn't know where the line should be drawn since there's no point of reference, and at that point it's pretty difficult to enforce things consistently, even if everyone wanted to.
The original proposal sounds feasible though, since that still has hard limits, but with room for people to use breaks differently. That way you wouldn't need to be as strict with draintime, while still keeping the intentions of the original rule intact.
The stuff about harder difficulties contributing more and hitsounding counting as contribution and all that would probably need a metric to be taken into account, since just leaving it up to the bns is going to lead to a lot of confusion due to everyone having different ideas of how much xyz contributes to a set. There are also people confusing effort for contribution so there's that too. Only problem with adding more metrics is that it'd get increasingly complicated, and somewhere along the line it'd get more complicated than it's worth.