Ruined Rex

After. The combat heal is part of the same cleanup that queues the trigger, and the trigger does not resolve until that cleanup has run to completion.

When Ruined Rex dies with lethal damage during a combat cleanup, the steps run in this order:

The pending trigger does not begin finalization until the cleanup task is complete.^[334] After the cleanup, the trigger is finalized and its target is chosen. Once no pending items remain, players receive priority before the trigger resolves.^[337.4] If it is not countered, it eventually resolves and deals 4 damage to the chosen enemy unit.

No. Ruined Rex chooses its target before Ferrous Forerunner creates its Mech tokens.

When both units die in the same cleanup, their Deathknell abilities become pending items.^[323.4] All pending items are finalized before players receive priority and ordinary abilities begin resolving.^{[337.3][337.4]}

Ruined Rex chooses its target during finalization,^[355.5] while Ferrous Forerunner creates its Mechs only when its ability resolves. Regardless of finalization order, those Mechs do not exist when Rex chooses its target.