I finally found something cleaner
Adding the web directory in the personnal directory of the user
mkdir -p /home/user1/websites/appOfUser1
chown -Rf user1:user1/home/devone/websites
chmod -Rf 770 /home/user1/websites
Then adding this line is /etc/fstab (install bindfs before)
bindfs#/var/www/application1 /home/user1/websites/appOfUser1 fuse force-user=user1,force-group=user1,create-for-user=www-data,create-for-group=www-data,create-with-perms=0770,chgrp-ignore,chown-ignore,chmod-ignore 0 0
And finally mount it!
it solves the concurrency problem, as every user in the equation (every developers and the web server) sees the permissions that he requires to operate properly and safely;
it solves the impermeability problem, as if you need one developer to access a particular application, you have to add a mountpoint in its home directory, but on the contrary, if the you need one developer to not access a particular application, you just have to not add such a mountpoint in it's home directory.
it solves the stability problem, as developers will never be able to change the file permissions set on the application files, as required by the web server to operate properly and safely.
best of all, it's simple to understand and to setup !