answer for question :

We can have multiple accounts on a node, but privacy of a transaction is on a per node basis and is independent on the accounts.
( in simple term , Every account created on the same node will be able to see the private transactions within the accounts of the same node. )

For example we create 3 accounts A, B, C on the node. Make a private transaction between A and B, In that case C will be able to see that private transaction as well.

Answer of part two would be, no there is no way to restrict deploying contract on a node.