Thank you for your question. I am very sorry for your loss. I apologize for the delay. I am the only European Law expert on the site, and one of my main clients has passed away this week, preventing me from following the questions.
The answers to your questions are:
1. If your mother passes away, you will be her heir together with him. Therefore, as a co-owner of the property, you may in your own name force its sale. Therefore, while he will be able to stall while you ask for a sale through the French courts, in the end he must sell it if you wish;
2. Yes, a Notaire's valuation is a valid method, in fact the valid method, apart from a judicial situation. The way the notaire made the valuation is not relevant outside of a court situation. However, you may contest the valuation, as per point 3.
3. Yes, you may organize a transparent valuation, in two ways:
-> either with informative value, which you could do through any local estate agents (i.e. based in the same city where the house is). There is no company to liaise with estate agents, you have to go straight to them or through a lawyer; or
-> with compulsory value.
To make a compulsory valuation you need, acting for your mother, to bring him to a civil trial asking for a forced sale. Within the trial you can ask for a court appraiser to look at the property. The appraiser is obliged to show his reasoning, and you may contest it. The final value is compulsory for both parties.
Regarding communications, sadly there is nothing else better than a lawyer (and since I am not available, please don't take this as self-promotion). Unless you have a very good and selfless friend to undertake this for you, a lawyer is your best option. However, language should not be a barrier. Here is a list of English-speaking local attorneys, compiled by the US Embassy: http://photos.state.gov/libraries/france/45994/acs/usc_paris-attorneys.pdf
I hope my answer was useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad