Can my mother enter Thailand on a 60-day visa and extend it for an additional 30 days before her return flight 90 days later?

I believe the requirement by Thai immigration is for onward travel, not necessarily return ticket. The traveler could purchase a refundable flight to a neighboring country. Also, if anyone asks, it will be the airline.

Please check the website of the Royal Thai Embassy in France website for detailed requirements and procedures for tourist visas.

Q1. will it be à problem having her return scheduled 90 days later when she is only given 60 to begin with?

-> The embassy in France accepts air tickets leaving Thailand within "less than 90 days", otherwise Single Entry Tourist Visa application may not be accepted. Remember the arrival date is day 1. 90-day counting is "89 nights", not 90 nights.

Q2. can she do the extension in Chiang Mai?

-> Yes, she can apply for a 30 days tourism extension at the Chiang Mai immigration office only once. Don't forget she get her TM30 receipt when she arrived CM in case.

Q3. how long in avance can I apply for her visa for z departure November 1st ?

-> SETV is valid for 3 months from the issued date to enter Thailand on it. But as the embassy in France states it might take 1 month to be approved once an application is completed. You can apply now and better to submit an application for SETV in early September at the latest.