"1m" is actually "1mm1m" - which means exactly what you said ("1" - original frame, "m" - interpolated one)
"2m" is "11m11" --> "1, 1, 2, 2, 2.5, 3, 3, 4, 4, 4.5, 5, ..."
"1.5m" - is like 1m, but with times closer to the original frames, something like "1, 1.16, 1.73, 2"
> is making 24 into 60 with the 1m frame interpolation mode the same as doing making 24 into 120 with the 2m frame interpolation mode?
definitely no