Sacrifice

No. Killing a friendly [Mighty] unit is an additional cost paid while playing Sacrifice.^356.7357.2 Countering the spell later does not refund that cost, so the killed unit is not returned.^{426.1.c426.1.c.1}

Yes, if Mordekaiser, Twice Slain is [Mighty], his once-per-turn replacement effect is available, and you kill another friendly unit for its cost.^371.1 Mordekaiser is healed, exhausted, and recalled, while Sacrifice's additional cost remains paid.

Sacrifice's additional cost attempts to kill Mordekaiser while the spell is being finalized.^356.7357.2 Mordekaiser's effect replaces his death, so Sacrifice's kill action does not occur.^370.1.a.1 The kill action being replaced does not undo its payment as a cost, so Sacrifice can finish finalizing.^357.2.a Killing the other friendly unit pays the cost of Mordekaiser's replacement effect, which then heals, exhausts, and recalls him.^203.1740.4.a

No. Your opponent has no normal reaction window after you choose the [Mighty] unit but before you kill it. Costs are paid during step 4 of finalization, and finalization does not pass priority.^357.2337.1.a During that step, as Sacrifice's controller, you may activate an eligible [Add] ability with [Reaction] when resources are required, even without priority.^357.1.a430.3 This limited exception does not create a normal reaction window or permit your opponent to play a spell like Stupefy between choosing and killing the unit.^430.2.a

Yes. The unit is killed while Sacrifice's additional cost is being paid.^356.7357.2 Its [Deathknell] is therefore added above Sacrifice as a pending ability.^808.1.d.2 The [Deathknell] resolves before Sacrifice draws and channels.^337.1.b340.1

Steps

1. Sacrifice moves to the chain as a pending spell.^354.2 Its controller then pays the additional cost by killing a friendly [Mighty] unit.

2. That unit's [Deathknell] is added above Sacrifice as a pending ability before the unit moves to the trash.^808.1.d.2

3. Sacrifice finishes finalizing.^359.3.a The pending [Deathknell] then finalizes.^{337.1359.3.b407.2}

4. The [Deathknell] resolves before Sacrifice. Sacrifice then resolves, drawing 2 and channeling 1 rune exhausted.^359.3.d