For your first question, did you consider the (direct) AF flight Lille-TLS at 13:15. I see a rate around 70 EUR for a o/w. Adding train Bruxelles-Lille and probably a taxi from Lille station to Airport should get you somewhere around EUR 100. This would be easier than flying anywhere else in Southern France and having to take a bus and/or train to Toulouse and faster than taking trains for the whole itinerary. Don't forget you have to change train stations in Paris (travel time: 30 minutes)
For your second questions, I second your idea of flying to Lyon and taking then a train to Chamonix. If you are on a real tight budget, you could take the night train Toulouse-Marseille and then TGV to Lyon. Costs about the same as the AF flight, but you'd save the hotel. Personally, I don't think I'd do this for the potential saving in question.