It makes me wonder if the episode might've been recoverable without all the trouble that swirled around its filming; keeping the romance subplot with Masters so they wouldn't have to stretch to fill time with nothing, not having the chaos of Barrymore's flaking out that forced them to rush in filming and made Brown's performance so harried. Would it at least be watchable without all the mess around it?
Probably not, since the script wasn't that much different in its earlier forms.
http://www.orionpressfanzines.com/articles/alternative_factor2.htm
http://www.orionpressfanzines.com/articles/alternative_factor.htm
And really, the romance subplot was almost beat for beat the same as "Space Seed," with the female officer being captivated by the charismatic visitor and seduced by him into betraying her duty. So it might've diminished "Space Seed" by making it seem repetitious.
Also, Brown's performance wasn't the problem. He just had fundamentally stupid and incoherent material to work with. I doubt John Drew Barrymore would've done any better, especially given his personal issues. (Has anyone seen him acting in anything from the period? How did his talent compare to Robert Brown's?)