Bernt,  My understanding is that you are right, that a properly
adjusted Duo-Art will not play 3-note bass chords at Accompaniment
zero level, and it should be adjusted so that it reliably plays single
notes with the dampers lifted.

I've seen a number of rolls, mostly from the 1970s or later, that have
code for chords that are too many notes at too low power to sound on a
well-adjusted Duo-Art.  Apparently, they were edited with the assumption
that most Duo-Arts are adjusted with the zero-level far too loud.  If
the customer really wants this roll to play, one option is recode the
roll by hand, cutting level 1 or 2 accompaniment codes at the
offending chords.

A few rolls with chords written this way were intended not to sound,
such as in Schumann's Carnival, where after some loud minor chords with
the sustain pedal on, a major chord is "played" silently to make the
dampers lift without the hammers striking the strings.  The chord is
heard only when the other dampers drop, and the sustained minor chord
resolves to a quiet after-ringing of the major chord

I've also seen your second issue, with the level 15 being quieter
than the levels 13-14.  If the problem were on the Theme side, I would
suspect a leaky "crash" valve that bypasses that knife valves at level
15, activated by the pallet valve and screw on the theme side.  But on
the Accompaniment side it is more interesting.

Have you checked that the spill-valve position is the same for level 15
activated on both the Theme and Accompaniment sides?  Maybe the spill
valve lever on the Accomp. side is over-rotating the spill valve rod,
and opening up the spill valve a little at the highest levels.

Best regards,
Ralph Nielsen
http://www.historicpianos.com/