Wow, sorry to read about your problems - truly awful! I don't know why they should be occurring and it all points to gross incompetence on behalf of the Notaire.
I bought a house in France last year and did not have a problem. The whole process should take between 2-3 months. The good thing about buying property in France is that (in theory) it is well regulated, with a clear procedure.
It makes me think that there are other problems behind the scenes, such as the seller's family, which the Notaire is dealing with but is not being professional in letting you or the agent know.
Do you have a solicitor of your own? I found this to be vital. They manage the whole operation for you and liaise with the Notaire and agent. There are a few legal firms in the UK with French speaking lawyers who will have dual qualifications i.e. they are both an English solicitor and French 'Avocat'.
I used a firm called John Howell who charged around £700 but it was well worth it. They even handle all your money transfers for you.
http://www.europelaw.com/Secondly - you are entitled to your own Notaire. The percentage fee (around 7.5% of purchase price) which goes to the main Notaire can also be used to cover a second Notaire. They would have to split the fee between them. This may help if the first Notaire is being sloppy. I suppose it’s a bit late now though, given that your Notaire will already have received the deposit.
At the end of the day the Notaire is a government official who is really just there to handle the bureaucracy and collect any taxes on behalf of the French state. He has no interest either way regarding the seller or buyer.
If he is being incompetent I'm sure the French authorities would be interested to hear about it, especially if you have already transferred the balance of the money for the house into his account. Your solicitor (if you appoint one) can advise you on all this.
There’s no way you should have been coerced into granting power of attorney to anyone either. Even if the Notaire is on the fiddle, I can’t see how delaying the sale to this extent is going to benefit anyone.
Finally, if you look at the clauses in the compromise de vente there should be some opt outs in case things don’t happen when they should. These are to protect the buyer and seller. There may well be reference to timescale for the sale. In my case the seller wanted a quick sale and I was committed to completing in two months, as set out in the compromis. Had I not done so I would have lost my deposit. There may be a clause which protects you and enable you to pull out and reclaim your deposit should the sale not happen within a specific period.
This whole situation sounds very dodgy to me and my instinct would be to get out and start afresh. My agent tells me that French house price inflation is slowing down as more houses come on the market, so you wouldn’t be missing out too much.
Sorry I can’t be more help but I strongly suggest getting a UK based solicitor on the case as soon as possible.
Bon chance!