3) It’s nonlinear, increasing methane concentrations also benefit methane scavenging bacteria which should increase the rate of breakdown as concentrations increase. OH only lasts around 1 second in the atmosphere so in theory it should be depleted by increasing atmospheric methane, but methane is only one of several things it reacts with so the difference is minimal without orders of magnitude change in Methane.
2) In terms of impact on climate the difference from 1990 to 2024 is mostly from CO2: https://en.wikipedia.org/wiki/Atmospheric_methane#/media/Fil...
3) It’s nonlinear, increasing methane concentrations also benefit methane scavenging bacteria which should increase the rate of breakdown as concentrations increase. OH only lasts around 1 second in the atmosphere so in theory it should be depleted by increasing atmospheric methane, but methane is only one of several things it reacts with so the difference is minimal without orders of magnitude change in Methane.