Does Adobe Audition use different modifier keys on Mac and Windows?
Yes. Mac uses Command and Option, Windows uses Ctrl and Alt, for the same actions. The key itself (letter, arrow, or symbol) stays the same; only the modifier changes.
Do Audition's editing shortcuts work in the Multitrack Editor too?
Not all of them. Adobe documents the "Repeat the previous command," sample-type, noise-reduction-profile, and channel-activation shortcuts as Waveform Editor only. Multitrack sessions have their own separate set of mixing shortcuts (knob adjustment, mute/solo/arm toggling, clip nudging).
How do I add a marker while editing in Audition?
Press M, or the asterisk key Adobe lists alongside it, at the current-time indicator. Ctrl+Alt+Left/Right Arrow (Cmd+Option+Left/Right Arrow on Mac) then jumps between the markers you have placed.
Is this the complete list of Audition's default shortcuts?
No. Adobe's own shortcuts page describes these as the shortcuts its team finds most useful, not an exhaustive list; the full set is visible inside Audition itself under Edit > Keyboard Shortcuts.