Mordekaiser, Twice Slain

Yes, if Mordekaiser's once-per-turn replacement effect is still available.^371.1 This includes another friendly unit that is about to die from the same combat.

Combat damage itself does not kill either unit. The kill action happens during the combat cleanup that follows, which would kill all units with lethal damage simultaneously.^{467.2.d468.1323.4370.1.a.2} Here, "simultaneously" describes the kills that cleanup is about to perform; it does not mean those units are already dead.

Replacement effects apply before the events they replace, so his effect steps in before the cleanup kills happen.^370.1.c At that point, the other friendly unit is still on the board. It may have lethal damage and be about to die, but you can still kill it to pay the cost of saving him.^740.4.a Paying that cost performs a separate kill action before cleanup performs any simultaneous kills left unchanged.^203.1373.1.a His own death is replaced, so he is healed, exhausted, and recalled instead.

The same timing applies to spells outside combat:

• Damage followed by a cleanup: If Singularity or Bellows Breath leaves him and another friendly unit with lethal damage, the cleanup waits until the spell finishes resolving.^321321.1 That cleanup would then kill them simultaneously.^{323.4370.1.a.2}
• One kill instruction: The Ruination and Cataclysmic Duel each have one kill instruction that can kill multiple units, so any deaths caused by that instruction are simultaneous.^370.1.a.2

Yes, if Mordekaiser's once-per-turn replacement effect is available and you apply it before Endless Riches whenever both effects apply, then kill another friendly unit for its cost. Mordekaiser is healed, exhausted, and recalled even if Endless Riches banishes that unit's card instead of killing it.

If Mordekaiser's card would go to your trash, both effects initially apply to his death, so you choose their order as his controller.³⁷² If Mordekaiser's effect applies first and you choose another friendly unit, killing that unit is the cost of saving him.^740.4.a If that unit's card would go to your trash, Endless Riches replaces killing it with banishing its card instead.^{370.1.a.1370.2} Replacing its associated game action does not undo the payment.^203.2 If Endless Riches does not apply to the chosen unit, that unit is killed normally to pay the cost.^203.1 Mordekaiser's effect therefore heals, exhausts, and recalls him.

If Endless Riches applies to Mordekaiser's original death and you apply it first, it banishes him instead of killing him, leaving no death for his replacement effect to replace.^{370.1370.1.a.1428.2.a}

No, because the kill used to pay Mordekaiser's replacement cost is attributed to his ability rather than the opponent's spell and therefore does not trigger Immortal Phoenix.

Replacing Mordekaiser's original death means the kill action from the opponent's spell does not occur.^370.1.a.1 Mordekaiser's replacement effect cannot apply again to the kill used to pay that cost.^370.2 You perform that new kill action as the cost, so you are responsible for it.^203.1412.1 Because Mordekaiser's ability contains the kill instruction, the kill is attributed to that ability and to Mordekaiser, not to the opponent's spell.^{412.5412.5.a429.5.b429.5.d} An Immortal Phoenix in that opponent's trash triggers only if they are responsible for a kill attributed to a spell they control.^{383.2.c.1412.6}

When Mordekaiser replaces the death, Deathgrip gives no might and you still draw 1. Under Deathgrip's erratum, its kill instruction is followed by a might instruction conditioned on "If you do" and then a separate draw instruction.

Replacing Mordekaiser's death means Deathgrip's kill action does not occur.^370.1.a.1 The conditional might instruction directly references that kill and therefore does not execute, while the draw instruction does not reference it and still executes.^359.3.e.14.b Killing a unit to pay Mordekaiser's replacement cost is a separate kill action attributed to his ability, not the Deathgrip action that was replaced.^{203.1412.5412.5.a429.5.b}