StuartV pointed me on 2 problems with the mod. The first one is fairly trivial- I managed to switch L and R channels. The fix as as simple as wearing the earbuds appropriately. Also this problem is unnoticeable because of the second one.
The second problem is serious- the stereo signal separation is almost nonexistent. The reason for this is that Sena uses differential amplifiers for the helmet speakers, and both "+" and "-" terminals have signals on it. It works fine for the speakers, because they have independent wires. Earbuds have one of the wires joined together. Normally it wouldn't be a problem, but when this joined common wire is connected to joined "-" wires of the amplifier, the severe cross channel bleeding occurs. Let me make it clear- this is not a problem with my mod. Sena factory earbud clamp suffers from the same issue. This is a design flaw.
There is one way to work around the flaw- to connect the common earbud wire to the ground. It fixes the stereo problem, but introduces a new one- an interference from the digital section of the main unit can now be heard as a buzzing sound in the earbuds. It is very noticeable during quiet moments, but is completely masked by a loud music or wind noise while in motion. I'd say if you listen to a lot of music this fix pays off, but if you are more interested in intercom, phone, audio books or talk show podcasts, you'll be better of with the degraded stereo sound without the background noise. It is also possible that the noise level may very between the main units, maybe mine is particularly noisy.
To use the common ground for better stereo it is necessary to remove the jumper from the "L-" point and to resolder a trace which was cut. If you start with an unmolested clamp, you just cut 3 traces instead of 4.
The restored trace is on the left
The final layout for improved stereo
This is a track you can play on your smartphone to check stereo separation and alignment
What Sena did with their earbud clamp is such a glaring design error I am not sure how it could slipped past their QA. I urge everyone to contact Sena and request a fix.