The unholy lodestone from a corpse cart as well as the 'master of the dead' effect from a necromancer are all different healing passives, all of whom can be stacked upon one another and can lead to some hilariously fast passive healing rates. This also means that a mortis engine itself will benefit from another mortis engine because it does not apply reliquary binding to itself. So yes that healing effect does in fact stack with regeneration and it does roughly double the healing rate of a Varghulf.
The mortis engine gives 'The Reliquary binding' effect to other units ( but not the engine itself!). Keep in mind that regeneration also means that the affected units get a weakness to fire and that is simply not the case with the mortis healing AoE. The Mortis Engine does not give the 'regeneration' effect to other units, it only applies that effect to itself. While you are otherwise correct, your example is not. But a Mortis Engine which gives the skill “Regeneration” to nearby troops won’t cause a Varghulf to heal any faster, because it already has that exact skill. For example, “The Hunger”, which gives regen in combat, and “Regeneration” stack.
