yes, it was a throwback to RE4 fending of an onslaught in the hut.
Yes, Survivor as DLC would be the right way to go. It seems obvious in one respect that the devs had looked at games like L4D, which is successful as well as Capcom's own RE Outbreak so maybe a game of this type had occurred to them at some point.
Total time survived will be dominated by cheap exploits. an OoB glitch for example.
A game type like this wont happen.
I say this, because RE4 did this type of thing the best and it wasnt turned into a game mode back then. i doubt it will be now.
Except that game modes like this have become very popular since back then, with modes like Halo's Firefight, GoW's Horde, and MW3's Spec Ops Survival. These all have survival as the main focus and even with score for the leader board as a primary motivator for some, score is most predominately determined by how long you live in these.
Now a total time survived could easily be exploited, which is why they should follow the lead of the AAA titles I just listed and do it in waves with predetermined (or close to in the case of GoW3) spawns based on level and wave. Require killing all enemies in a wave to move on, use Mercs scoring minus the combo bonus, and you've got a good survival mode with how long/many waves you survive being the biggest factor in score.
They can have Zombies, etc..., become more aggressive as time lapses. Plus they can spawn more Zombies by time as well until you hit a max cap, ie 100. This way you will not be able to leave only 1 Zombie and run from it forever.