Back in time BOB killing needs to be tested. Two ways: He's not there, keep moving and kill 3rd spawn. Go back and see if 1rst spawn is there.
And, 1rst Spawn, kill him. Progress through level, go backwards. See if he spawns again.
Here's a rendered video of one of the Oni Bob Spawns.
To test your back-in-time scenarios, we need to pin down a level where it is even possible to "backtrack" far enough to load an area where BOB previously spawned. Winter Contingency is probably the easiest, but I have to double-check. ONI won't work because you have to go up the elevator to unload the courtyard area where the first BOB spawn point is- and once you go up that elevator, you're not going back (unless you restart the mission... and then all bets are off.)
What CAN be done on ONI is to open the doors to the base (which loads the BOB area) and then dart back out of the doors, hop in the Warthog, and unload the area. I did this many times (and IIRC when the BOB spawned there, he was the same every time the area reloaded... but need to reconfirm). The trick is knowing whether or not the first spawn point is being used. I believe that once you proceed far enough to be able to kill the BOB, the doors close behind you and cannot be reopened. So it's probably impossible to kill the ONI BOBS multiple times without restarting from a previous checkpoint.
Here's the testing plan for getting the courtyard BOB to respawn:
1) Start Rally Point Bravo on ONI: Sword Base from the campaign menu
2) Proceed to the courtyard and confirm that BOB spawn point 1 is being used (i.e. the BOB appears)
3) If BOB does not spawn in the courtyard, repeat steps 1 and 2 until he does.
4) Once BOB is chosen to spawn in the courtyard, he will ALWAYS spawn in the courtyard until you go back to the main menu and restart the mission. Choosing "Restart Mission" from the pause menu does not seem to initialize the logic to select the BOB spawning location. Therefore, select "Restart Mission" to begin again from the Rally Point Bravo checkpoint, knowing that BOB will be spawning in the courtyard.
5) Activate the switch to open the doors to the courtyard and walk in.
6) Wait to see enemies appear (when the elites and grunts spawn, so does the BOB)
7) Quickly run back through the doors, hop back in the Warthog, and drive back towards the starting area. This will unload the courtyard.
8) Return to the switch to open the doors and activate it. The courtyard area will reload.
9) Repeat steps 4-8 multiple times to ensure that you get a decent sample size of BOB respawns (2 is not enough!)
10) Exit back to the main menu, save the film, view in Theater to see the results. Did the BOB respawn in the same armor every time?
I didn't try backtracking in Nightfall. Should be possible based on the layout, but I haven't tried.
Plenty of SGP homework here!