Order of execution is never guaranteed if you do it on independent key-presses. If you have a compound action, what I would recommend is to actually make a batch file that runs both commands through the commandline, that way its more likely that the first is complete before the second runs.
The reason these commands are run through the launcher is that while MCC commands can take parameters, passing anything more complex then a number is complicated (like a filename, "selected" is just a special case in the same logic). And most MCC commands exist because MC uses them to communicate between components - for this purpose we already have MCC_PLAY_ADVANCED, but its parameter payload is complicated and can't be fed from a keybinding.
"play all selected files", "append all selected files" etc in how many variants of playback methods there are seems extremely specific for MCC commands when you can already do these through the launcher, which is a more versatile interface for complex commands with parameters, and doesn't require making a lot of special cases.
In summary, MCC is mostly designed to carry two numbers, the command and one numeric parameter. More complex parameters require an extreme level of complexity .. or using the launcher, which can translate for you. Thats what its for.
The launcher can also run MCC commands already, eg. "MC29.exe /MCC 22001,2" should work fine.
