Each digital contract that is issued by a publisher is added to the participant based blockchain. You can always access your digital contracts by logging into your blockchain node and examining any existing contracts in-place for their validity with your clients
Absolutely; the drdm.io service is designed to allow you to maintain contracts with all of your clients as well as maintain your data compliance obligation to any sources of data that you redistribute. The data owner will be able to see a map of which clients are contracted for their data as well as how many clients you redistribute your data two (but your clients identities remain confidential to you.)
You can run as many nodes as you feel makes sense for your organisation. You may want a couple of node per location or you may just want to run cloud deployed nodes. You can always rebuild all of your data from the blockchain and setting up a new node is very easy.