When using sync, I can define a partition id. This is how I intend to separate tenants. Its honestly one of the best solutions I have seen for a multi-tentant cloud database (unless of course I am missing something).
Client archive options are a good idea, but most people will likely not use it unless forced to do so. Meanwhile, I (as a mobile developer) would be responsible for the data transfer rates of these large databases to each device. I am just concerned about an app not properly thought out causing large database utilization fees.