Finally, is it possible to limit the amount of cache a particular
backing device can use?

I'm not sure if it is, but you might be able to get the same end result
by using partitions on the cache device to make separate sets: That is,
instead of:

   { cache: backing }
   { sdb  : sda1 }
   { sdb  : sda3 }

You could try

   { sdb1 : sda1 }
   { sdb2 : sda3 }

and so forth, partitioning sdb appropriately.
[ Disclaimer: I have not actually tried this ]