I've done a winter season in Morzine, but was lucky enough to know someone who used to run a seasonaire chalet so didn't have to go through the hassle of finding somewhere to stay. However I did find out the people I shared with, did was use sites like
Natives
or may be something useful here:
Morznet
The other thing to do would be to contact chalet companies that operate biking holidays in the summer (e.g. Endless ride, Flow). They may be able to put you in contact with someone who rent apartments. They may have live out staff in the summer who also need places to live.
Also may be worth contacting comapnies who run winter seasonaire chalets, as there is a chance they may, or at least know someone who rents in the summer.
No idea about the taxes and stuff. I didn't pay any directly, but it may have been included in my rent.
Hope some of the above useful.