Organteq sounds amazing in stereo, but I have noticed that many of the stops lose power in mono (left + right).

For example, try using just the Piccolo 1' stop. In stereo (or when listening to just one channel), the volume is roughly consistent across the keyboard as you would expect. But if you switch to mono, the volume varies by about 12 decibels depending on which key is pressed. I tried changing polarity or adding delay to one channel, but it just changes which keys are affected.

In the real world, a large minority of listeners are using mono systems, and they always sum the left and right channels. This gives me two options: release a stereo recording that sounds great for most people but absolutely terrible for mono listeners, or release a mix with only the right channel which loses all stereo width but sounds much better in mono. Neither option is great. My best compromise so far is to take one channel and use third-party stereo wideners which don't seem to cause any phase issues.

I'm wondering if anyone has found a fix. Is there a good way to retain Organteq's stereo width without abandoning mono listeners?

A messy option might be to record a mono version from the stereo version playing on your pair of speakers; that is acoustically, with a microphone. The problem is probably caused by phase differences, but have you tried setting stereo width to zero?
jzz
Edit:
I misunderstood your problem: you need a stereo recording which will behave nicely when 'mono-ed'. Suggest same solution, but record stereo and check that version's behaviour.

For example, try using just the Piccolo 1' stop. In stereo (or when listening to just one channel), the volume is roughly consistent across the keyboard as you would expect. But if you switch to mono, the volume varies by about 12 decibels depending on which key is pressed. I tried changing polarity or adding delay to one channel, but it just changes which keys are affected.

In the real world, a large minority of listeners are using mono systems, and they always sum the left and right channels. This gives me two options: release a stereo recording that sounds great for most people but absolutely terrible for mono listeners, or release a mix with only the right channel which loses all stereo width but sounds much better in mono. Neither option is great. My best compromise so far is to take one channel and use third-party stereo wideners which don't seem to cause any phase issues.

I'm wondering if anyone has found a fix. Is there a good way to retain Organteq's stereo width without abandoning mono listeners?