gives the Champ controlled by the player extra sustain and AoE damage / lifesteal.
gives the Champ controlled by the player HP, AoE of different shape, and damage scaling off of HP.
Both items simply change the way the Champ itself performs without much relation to the enemy targets. The two items both focus more on the champ that they are on.
IS a counter to enemy sustain on top of being a counter to armor stacking.
IS a counter to enemy HP stacking on top of being a counter to armor stacking.
These two items are not as dependent on the Champ they're on, but who they fight. In other words, they are counters.
Items built with a focus on the player's Champions work very differently from items built with a focus on the ENEMY's Champions.
I am of the opinion that items that are situationally built to counter the enemy's Champion +build should NEVER be easily interchangeable, especially when the items singlehandedly act as counters.
At least
requires more damage and / or lifesteal to amplify its usefulness while
requires HP stacking from other HP items to maximize its usefulness.
and
don't require any such synergy with other items to counter the built lifesteal / stacked HP.
TL;DR Each set of items mentioned operate in entirely different ways. They should not be compared, and Mortal Reminder / Lord Dominick's should not be interchangeable.