Meanwhile I got that same mirror, tried it on my trekking bars, on my wife's bike (not trekking bars), and eventually gave them to a friend. Ended up with an eyeglass mounted mirror that I love.

I had the same problem with mounting stuff to the trekking bar. Some solutions I tried with varying degrees of success: mounting my light upside down, so it could occupy some of the same space as the bell; Mounting the speedometer to the stem; getting a spacer-mounted bell that goes between the headset and the stem (didn't use that solution with my trekking bars, but it's probably still an option with them).

I've considered on of those accessory extenders and/or a 2nd, lower, stem with a shortened, flat bar for lights, but at the moment I've managed to relocate most of my stuff out of the way.

I had brake/shifter combo units like yours, and I kept feeling that they were not the best option for trekking bars, but I never tried any alternatives, either. I would expect the grips to be flipped, though, with the wider part supporting the meatier part of your palm.