Some thoughts...
RDuration 10 means a random, max 10% change in the duration. As always, MMA uses percentages.
However, according to the docs, RTime 10 means a random change of max 10 ticks. Really? Or should this have been percentage as well?
Using a % here makes little sense to me. A % of what would be the question. At this point MMA has no idea of the note duration, just a start and end point in tick offsets.
With RDuration and RTime (and Articulation set to 100 (percent?)), is it possible that notes will overlap?
Absolutely. Proceed with caution.
Have you considered
RTempo to make small changes to the tempo, as normal musicians do?
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Actually had it running in an early test. IIRC, it reminded me too much of a grade school band
Where would the variance be applied? On each bar, note, or? Tempo is a value inserted in the META track and used by the player. The individual tracks don't have a concept of Tempo. So, it's possible to insert a bunch of timing values that the player can follow, and that's what things like "Tempo +20 4" (increase tempo by 20 BPM over 4 bars) does ... again, IIRC this command inserts a new tempo value on every beat.