Sacrifice

No, as the kill is an additional cost and countering a spell never refunds costs already paid.

Killing a friendly Mighty unit is an additional cost required to play Sacrifice. A countered spell does nothing and is not considered to have been played.^{[425.1.a][425.1.b]} Countering does not refund any costs paid to play a card, including additional costs.^{[425.1.c][425.1.c.1]} That cost is paid before the spell resolves, so the killed unit is not returned.

No. The unit is not a target, and finalization does not pass priority, leaving no window to react to either the choice or the kill.

The unit killed as a cost is not a target — objects referenced as part of a cost are explicitly excluded from targeting.^[355.10.c]

Costs are paid during step 4 of finalization, and finalization does not pass priority.^{[357.2][337.1.a]} An opponent cannot play a spell like Stupefy — which reads "Give a unit -1 Might this turn, to a minimum of 1 Might" — to reduce the unit's Might and make it no longer Mighty before the cost is paid. The first opportunity for either player to act is after Sacrifice is fully finalized, by which point the cost has already been paid and the unit is already in the trash.

Yes. The kill is a non-standard additional cost, not a resolution instruction, so it happens during finalization. That puts the Deathknell on top of Sacrifice on the chain, where it resolves first.

The opening line of Sacrifice — "as an additional cost to play this, kill a friendly Mighty unit" — is a cost, not a line of rules text that executes on resolution.^[356.6] Costs are paid in step 4 of the play process, before Sacrifice has even finished being finalized.^[357.2]

When a unit with Deathknell is killed to pay that cost in step 4, its Deathknell ability is added to the chain as a pending item before the unit moves to the trash.^{[428.1.a.1.b][808.1.d.2]} At this point the chain holds two pending items: the Deathknell above, and Sacrifice below, still mid-finalization.

Sacrifice then continues through step 5 (legality check) and step 6, becoming a finalized chain item.^[359.3.a] Because the Deathknell is still a pending item at that point, it completes its own finalization steps before play proceeds.^[359.3.b]

Once both items are finalized, the chain resolves from the top down. The Deathknell was added last, sits on top, and resolves first. Only after it fully resolves does Sacrifice resolve — drawing 2 and channeling 1 rune exhausted.