Just to follow up on my own post, comments in these two threads relate to my G15 broadcasting issue:
[Octopus] G15 compatability (related to Octopus)
G15 Keybord setup guide (Dirjax's comment)

So it seems others are having similar issues: the correct combo is read by one or more toons, but not by all. I read in that thread that Logitech's software differentiates between left and right CTRL and ALT - that might be a part of the problem; could there be a potential issue and workaround using Keyclone? Then again maybe it's a Trinity Bar problem.