Gby,
First I want to comment on your scope. The pictures look like they came from mine, a TDS-694C. But with the same 10G sample rate as yours, mine yields a 3G bandwidth.
Anyway, I think your analysis of the modes is right. However, I think the reason you are seeing slightly different rise times between built in and TrueArb is due to slight measurement error.s. It is possible that the difference in frequency and/or duty cycle causes the peak to peak voltage to be slightly different, and that mucks with your rise time measurement slightly.
Anyway, I just got my SDG2042X and am having fun playing with it, too.
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