sudo, yes, the STIs would need to go AFAIU. The trekking bars diameter is the same as MTB handlebars. Lots of people swap the stem when they get trekking bars. Since the reach is different on them than on other bars. For me, I liked the new riding position afforded me by the new bars, so I still use my old stem - a longer adjustable one that came stock on my hybrid.

Yeah, if you go IGH, I'd think you should easily be able to find shifters that would fit the diameter of the trekking bar.