The most likely reason it doesn't keep up during warmer weather is that fin-tube baseboard output is extremely non-linear with water temp at temps below 110F. This is because the convector height is only ~6" with baseboard, which isn't enough stack-effect to move much air volume when the fin-tube is at bath-water temps.
Setting it up the curve to work off the return water temp is probably going to fix that issue. But rather than 120F, set it at 100-105F. That is plenty high enough so that the aquastat on the Myson will trip, and under sustained calls for heat you'll still be putting out sufficiently high temps to cover the load while still condensing, assuming it still allows the +15F overshoot while cycling the way it did on your prior curve tuning experiments. If you set it up at 120F it'll only be condensing half the time as it cycles, since anything more than a 5F overshoot would take it over the condensing zone.
Do you have any beta on why the unit is being recalled/replaced?