@trunolimit
Contact your reseller, they should be able to add historical purchases into DEP for you. You will need to provide them with your DEP ID.
Once you have this setup make sure that any future devices are enrolled at the time of purchase.
Going back to your first question you can either allow / block apps but you cannot do this on a timed basis its an all or nothing approach. If you are wanting to block after a certain amount of time you would need to look for a content filtering system that allows you to setup quotas.
Good luck and if you have anymore questions just ask.