Basically, yes - you should be able to convert deployments into child cells of a parent (call if "API") cell.

Set up the new "API" cell, configure the child cells with the right keystone things and nova-cells configuration, then point your horizon at the new API cell. It won't be a pretty conversion process, but it should be possible to hack it through.

Each cell does indeed have its own database, queue, scheduler etc, so they're fairly independent. A failure on one cell should not kill the other cells.

At time of writing, you cannot live migrate between cells. However, if you add in appropriate availability zones (which are shown to users) it enables your users to have apps that can work across multiple availability zones (map 1:1 with cells, perhaps) and allow them to work around the failure this way.