Is it possible that binarual beats are simply triggering complete psychical effects within the brain and can be explained scientifically? Meaning that Robert A. Monroe was simply having many trippy fun loving hallucinations additionally fueled by a powerful imagination?

No.
You'll find far more info on this in Bob Monroe's books, plus all the documented record of the Monroe Institute.
Be aware that the Monroe Institute has on its board of governors (or equivalent) more doctors and psychiatrists than you can shake a stick at, and they've all got degrees. Some of them are well known around the word for their research -Dr. Charles Tart is possibly one of the most eminent.
These folks wouldn't waste their time with 'trippy, fun loving hallucinations additionally fueled by a powerful (some detractors would say overactive) imagination'.

Best wishes,
David.

Monroe's own experiences were not triggered by Hemi-Sync, which was not developed by him and the engineers who worked with him until the 1970s. His OBEs began in the 1950s. Read his first two books, Journeys Out of the Body and Far Journeys; it's all in there. Certainly there are changes in the brain activity of those who use Hemi-Sync. Do those physical changes produce (cause) the experiences, or are they simply correlated with them because they permit those experiences to occur? I would say the latter. But as Monroe said time and again, the best evidence is experience itself.