"Is it a promotion of a monoculture if Microsoft are getting a seat at the Chromium table, not just Google?"
Yes. The more companies that sit at that table, the more of a monoculture we have. The diversity that's important isn't in the number of companies using the same engine, it's in having several different engines.
"I can't imagine there's many webdevs saying "oh, no! Fewer browsers to support!""
I'm sure you're right, but let's talk frankly here -- as an end user, I don't give two shits about how hard the life of web developers is, just as so many of them don't give two shits about me.