Rampage has no cast time and does not interrupt Hecarim's previous orders.
The stacks apply to Rampage's base cooldown (meaning their reduction applies first, and only after that does the one from items, runes, and/or masteries take effect) meaning the lowest it can get is 1.1 seconds (if powered by 45% cooldown reduction and both stacks)
Spirit of Dread has no cast time and does not interrupt Hecarim's previous orders.
The only restriction Spirit of Dread has for healing Hecarim is that the source must not be an enemy (damage from neutral monsters will count) and that enemies spending their own health to use abilities will not count.
The healing cap against minions accounts for every single one regardless of how many are present and/or affected by Spirit of Dread at the time of cast.
Active:Hecarim is temporarily ghosted and gains bonus movement speed for 4 seconds, beginning at 25% and increasing to 75% over 3 seconds.
While Hecarim is charging, his next basic attack gains bonusrange, knocks back his target, and deals physical damage. The damage and knockback are increased based on how far Hecarim travelled during Devastating Charge's duration, ending its effects in the process.
Active:Hecarim summons spectral riders and dashes forth with them in a line, becoming immune to crowd control while charging and dealing magic damage to all enemies in their path. Enemies within range of where Hecarim stops also flee from him for 1 second.
Magic damage: 150 / 250 / 350 (+ 100% AP)
The wave of spectral riders travels through Onslaught of Shadows's full length, regardless of where Hecarim himself stops.
Spell shields will block the fear but not block the spectral riders' damage.
The flee will only be applied to enemies near Hecarim when he finishes charging and not to those only hit by the spectral riders (but their damage will still apply to feared enemies)
Unlike fear the flee from Onslaught of Shadows will always make affected units travel away from Hecarim (even as he continues to move himself) over its duration (its Loss of Control Indicator icon is different in comparison)
As a result Hecarim has some degree of control over where affected units will run.
While charging Hecarim gains 100% Tenacity but he can still be damaged.