How can I set an alarm for the first Monday of each month?

You didn't mention whether you are willing to buy an app to do this. I recently got an app called Alarm Plus Millenium that I see can do this (and lots more). I mainly got it for its task manager abilities, but I also needed a newer alarm clock app to replace my old unsupported Gentle Alarm, and this app was the best I could find for what I wanted.

Also, FWIW, I've had some email exchanges with the developer, and he was very responsive, quick to reply, and although in Paris, was very proficient in English (my only language).

Just saw your question about setting an alarm for Calendar events. Yes, Google Calendar allows you to set all sorts of alarms for the events. You can set more than one if you want.