Why highly unstable cyclic boron compounds should be stabilised by two electron reduction?
Your energy diagram is that of a linear moiety, like an allyl cation. For a cyclic structure, it would be somewhat different, but the general idea remains the same: there is one $\pi$-bonding orbital, and when two electrons go to it, everyone wins.

As for vibrational spectroscopy, well, it can help you tell the cyclic structure from linear, if that's what you want.