Gamma's only means of destroying anything is with their lock-on. I can't claim to know if you've played Gamma in SA1 before, but for the record, Gamma could lock onto and destroy some environmental objects in that game, too, to no purpose. (Although I at least agree that Techno Hill Act 1's flowers should probably be removed from the list of objects they can lock on to, with how much they clog things up.) The mines and barrels are absolutely valid as lock-on targets, both because they give time and because this gives Gamma a safe way to deal with them from a distance- seeing as that's kinda their schtick. And I think the stalactites are uncommon enough, by comparison to TH1's flowers, to stay shootable. Iirc the stalactites even have some collision, so destroying them could hypothetically help you move around.
As for the monitors, I must reiterate locking on to them is the
only method Gamma has to destroy them. That and they're generally very helpful to destroy in the first place, save, say, losing a magnet shield, or if they're eggman monitors- but that encourages being at least a little careful with your locking on, which I think is perfectly reasonable. Also, in SA1, monitors could simply be walked through by anyone to gain the benefits within, but Gamma could lock on to them there, too. (There's actually
a mod that converts all monitors to Adventure-styled ones if you're interested in being able to walk through them in SRB2 as well.)
There are, memory serving, plenty of environmental objects that Gamma doesn't lock on to as well, like the trees and flowers in Greenflower for example. All this to say, any 'errors' in the lock-on system are almost certainly intentionally recreating behaviors from SA1.