A
Two clearly labeled sections
"Filter to…" on top, "Mark selected as…" on bottom. Selection count shown as a section meta.
B
Two-column "Filter | Mark As"
Side-by-side columns share the same icons. Eye-tracking left for filter, right for action. Compact.
C
Mode toggle on top, then a single icon row
A compact pill-toggle ("Filter" ↔ "Mark As") flips the verb. Below: one row of icons, big and clickable. The selected count is the trigger badge. When nothing is selected, "Mark As" tab is disabled.
12 selected · "Mark As" tab
No selection · "Filter" tab
My pick: A — labeled sections beat hidden modes. The user always sees both verbs at once, no toggling, no "is it filter mode or mark mode" confusion. Selection count appears in the section header so it's obvious which half acts on what. Tri-state counts ("3 / 12") on the action rows mirror the right-click context menu pattern you already use.