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 has 325 range, knocks back his target 250 / 450 units, and deals physical damage. The damage and knockback distance are increased based on how far Hecarim travelled during Devastating Charge's duration, ending its effects in the process.
Minimum Physical Damage:
45 / 80 / 115 / 150 / 185 (+ 50% bonus AD)
Maximum Physical Damage:
90 / 160 / 230 / 300 / 370 (+ 100% bonus AD)
If Hecarim's target is beyond narrow terrain, Hecarimdashes over it to reach his target.
Active:Hecarim summons spectral riders and dashes, unstoppable, forth with them in a line, dealing magic damage to all enemies in their path. Enemies within range of where Hecarim stops are also made to flee from him for 0.75 - 1.5 (based on distance traveled) seconds, also gradually slowing them by up to 99% over the duration. This slow cannot be reduced.
150 / 250 / 350 (+ 100% AP)
The wave of spectral riders travels through Onslaught of Shadows's full length, regardless of where Hecarim himself stops.