A thought on the Neutralizer specifically: effectively, it could be argued that it's a soft trap - it doesn't KILL the lemming, you can still save it, but if you can avoid it you probably don't want your lemmings being neutralized. With that in mind, would it count as a trap when interacting with a Disarmer?
Regarding triggers for both types: Infinite, constant seems like the best choice: while both infinite triggered and single-use also have interesting puzzle potential, I think the constant is the most important of the three to have - single-use by itself in particular by itself would be bad, because if you wanted more, if not explicitly banned by the engine it would strongly encourage designs that stack several single-use ones on top of each other, and if you wanted to do it fairly, well, you'd need a lot of space, and it's not going to look nice, as well as being time consuming to bypass if bypassing them is part of the puzzle. Infinite triggered has already been ruled out, but similarly, if you don't want infinite triggered neutralizers to be bypassed via Compression Method tactics, you have to stack them. Thus, I'd put the priorities like this:
Most important: Infinite, constant
Still interesting: Infinite, triggered
Don't do this one unless you also include both the other ones: Single-use
ALTERNATIVELY:
Have one version of the object (constant effect) that has a lemming count similar to how exits work in the sense that it can, but isn't required to, be infinite.
For the record: At this time I support the Neutralizer and Deneutralizer, but not the Zombifier. If I were forced to pick one I'd learn towards the Neutralizer, but would prefer having both the Neutralizer and Deneutralizer. Given the existing zombie mechanics I am not convinced the Zombifier brings anything interesting to the table, and it's another object to design (I mean, maybe the code could be shared with the Neutralizer or with existing zombie mechanics, but it's still another object that needs to be visually designed).