The main reason trying to modify an event’s low level channelgroup, is that the event system is already setting that stuff itself. Using the low level API assumes you know that the higher level event system will be manipulating everything to get it to sound right.
In this case it will be setting the format of the channelgroup , as set by the track format in the UI (ie in the effect deck, you can select ‘mono/stereo/surround/auto’ for input format or output format when right clicking on the VU levels.
Trying to change studio’s signal speaker mode format will usually just have studio API trying to force it back to what it expects it should be.
You can insert extra DSPs in the chain, and do down/upmixing there, as long as the output is what studio expected fromt he sound designer’s point of view.