Ruined Rex

Ruined Rex's [Deathknell] deals damage after the surviving units are healed. The combat cleanup adds the ability as a pending item.^808.1.d.2 That cleanup heals all surviving units before the pending ability can begin finalization.^{468.1.a.1320334.2}

If a legal enemy unit is available, the trigger's controller must choose one as its target during finalization.^{355.10403.2403.4.b} Once no pending items remain, the controller of the next item receives priority before the trigger resolves.^337.4 It deals 4 damage to the chosen enemy unit.

No. The trigger's controller chooses its target before Ferrous Forerunner creates its Mech tokens.

When both units die in the same cleanup, their [Deathknell] abilities become pending items.^808.1.d.2 Each pending item finalizes or leaves the chain before priority is awarded.^{337.1337.3337.4}

The trigger's controller chooses its target during ability finalization.^355.10403.2 Ferrous Forerunner creates its Mechs only when its ability later resolves.^340.1 Regardless of finalization order, those Mechs do not exist when the trigger's controller chooses its target. That choice cannot change when Ferrous Forerunner's ability later resolves.^355.15