WISEe: I run OBS on a Mac and I find the UI to be generally flakey and unreliable. It is usable, although I am not able to configure the various windows as I would like. As far as missing the Start Recording button, do you see the Controls section at all? If not, is the menu item Docks => Controls checked? Have you tried resetting the UI? Also, on my Mac, I have found some apps display pop up windows on a second monitor.
I hope that helps.
Tony N