Can "enemy" be used as a plural noun? (Not as a collective noun)

Yes, you can use enemy with a plural verb, but when you do so it is only ever as a collective or mass noun, never as a countable one. The paywalled OED has this as its sense 3a:

3. a. The hostile force. Originally only as quasi-personified, with agreement in singular; now also as collective with agreement in plural. Also, a hostile ship.

So you could certainly say that the enemy are upon us now to mean the whole mass of them, but you cannot then count them by saying ^✽five enemy to mean five of the enemy. Therefore you may not then use it with quantifiers; only partitive constructions are then allowed, just as with any other mass noun whether singular or plural.

This is not actually an irregular plural. It’s just something you can do with collective nouns in English, like when the jury are still sequestered in deliberations. That is not irregular.