Yes, it’s true that Glen Campbell was the “fifth Beach Boy” on the road for a few months in 1965
There’s a theory out there that Brian essentially faked his late ‘64 “breakdown’ because he was tired of touring and live performances and just wanted to stay home and make records in the studio. Hard to know the truth with such a complex character.
There’s a funny story told by a lawyer who worked with the Beach Boys in the ‘70s. There was a big meeting of the band and management and lawyers, and some issue came up for a vote. Meanwhile Brian was sprawled out on a conference table taking a nap. Attempts to snap him out of his slumber had limited success. Finally, exasperated, someone said, “Brian, if you can hear me, we’re voting on whether or not to do {...whatever...}. If you vote yes, just knock on the table once. If you vote no, knock on the table twice.”
To everyone’s surprise, Brian, still apparently snoozing on the conference table, proceeded to knock on table... three times.
Parallels what happened with XTC's Andy Partridge.