Thanks for you question!
First about Zeb.
Zeb's trait works the same as saying "Zeb has a minimum bullet rating of 2, is a stud, and cannot be changed to a draw, while opposing a dude with grit 11+".
So with keeping this idea in mind, let's track through each of the actions in the sequence above.
Zeb is opposing a dude grit 11+. So Zeb is a stud and has 2 bullets.
Opponent plays SIYE. Zeb receives a -2 bullet penalty, however the minimum bullet rating is 2, so Zeb's bullets stay at 2. Zeb would be changed to a draw, however the trait prevents this. Keep track off both of these effects, however.
3A. You play FoTD. This gives Zeb +1 bullets, and gives an opposing dude -2 bullets. If the -2 Bullets you give to an opposing dude means there is still an opposing dude with 11+ grit, Zeb's trait is still in effect. So Zeb currently has (-2 + 1) = -1 bullets. But since Zeb's minimum bullet rating is 2, Zeb's bullets stays at 2.
3B. You play FoTD. This gives Zeb +1 bullets, and gives an opposing dude -2 bullets. If the -2 Bullets you give to an opposing dude means there is no longer an opposing dude with 11+ grit, Zeb's trait is not in effect. This mean Zeb is a Draw and currently has (-2 + 1) = -1 bullets. So Zeb is a 1 Draw now.
The key thing here is you keep track of any modifiers when computing that running total, when considering what is the current amount of bullets, value, grit, influence, etc..,
Regarding this question: "? So, if I somehow give Zeb a -1000 bullet modifier while his trait is active, is his trait giving him a +1000 bullet modifier to balance it?"
No, that's not how things work. Zeb's trait is behaving as a minimum amount(2), instead of typically the 0 that we use for minimum amount we use for bullets or influence, or 1 for value or grit.