Alcohol is legal
Can soldiers get pissed on guard?
At my first unit the guard commander got smashed out of his mind on southern comfort. He then threatened the guard holed up in the guardroom (armed but unknown ammo state - he was one of the ammo storemen) and was eventually "subdued" courtesy of pick helves , sedated by the MO and was eventually faced some "disciplinary action".
So yes, clearly people can get pissed on guard.
Every time I covered someone else's duty it all went tits up, unit barman went awol, a Sgt went awol and was rumoured to be armed, Woodbridge tried top himself, and Dumpy shot at the gas(oil?) tank by the workshops.