When using Arma 3 Apex keyboard preset, you can select "Manual fire" in Rhino MGS (UP) and T-100 Varsuk, when turned out as commander, but you can't manually shoot with LBM (left mouse button) - nothing happens.
https://www.youtube.com/watch?v=VeULs6mb11M
When using Arma 3 Apex keyboard preset, "Manual fire" works fine on M2A1 Slammer, M2A4 Slammer, MBT-52 Kuma, T140 Angara and T140K Angara.
"Manual fire" works fine on all tanks, only when using Arma 3 keyboard preset (non-Apex).
Arma 3 and Arma 3 Apex keyboard presets, both use same key to manually fire - LMB (left mouse button).

When using Arma 3 Apex keyboard preset, you can order the AI gunner to fire from 120/125 mm cannon in Rhino MGS (UP) and T-100 Varsuk, when turned out as commander, but it won't fire with Ctrl+LBM (left mouse button) - nothing happens.
https://www.youtube.com/watch?v=7XhtJ5Eau60
When using Arma 3 Apex keyboard preset, ordering the AI gunner to fire from 120/125 mm cannon works fine on M2A1 Slammer, M2A4 Slammer, MBT-52 Kuma, T140 Angara and T140K Angara.
Ordering the AI gunner to fire from 120/125 mm cannon works fine on all tanks, only when using Arma 3 keyboard preset (non-Apex).
Arma 3 and Arma 3 Apex keyboard presets, both use same keys kombination to order the AI gunner to fire from 120/125 mm cannon - Ctrl+LMB (left mouse button).