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.^[335] Only then does it enter the FEPR process, get finalized (targets chosen), and resolve — dealing 4 damage to the chosen enemy unit.

No. Targets are chosen during finalization, and Ferrous Forerunner's Mech tokens are not created until its Deathknell resolves — which only happens after all pending items have been finalized.

When both units die in the same cleanup, their Deathknell triggers are queued as pending items simultaneously in turn order,^{[323.4][383.3.d.1]} and items finalize in the order they were appended.^[337.1.b] No item resolves until all pending items have been finalized.^[337.1.c.3]

If Ferrous Forerunner's controller is the turn player, their trigger is queued first and finalizes first. The ability does not resolve yet and no Mech tokens are created. When Ruined Rex's trigger then finalizes and must choose a target,^[355.5] Ferrous Forerunner's ability still has not resolved, so the Mechs do not yet exist on the board.

If Ruined Rex's controller is the turn player, Rex's trigger is queued first and finalizes first. It must choose a target immediately^[355.5] — before Ferrous Forerunner's trigger has even finalized, let alone resolved. Again, no Mechs exist.

Either way, once Ruined Rex's trigger has finalized, its target choice is locked in and cannot be changed.^[355.15]