I’m not sure what eMPA is, or if it it’s a general term or a specific system. If the latter you should be sure to consult any specific documentation for that system.
Q1 “should it be the only given drug & dosage”
You need to create a full resource, which includes as much detail as you can give, and can “stand alone”, as much as possible. Of course you need things like the patient and the time. So it is not “only” drug and dosage.
Q2 POST creates a new resource, PUT updates an existing one. You want a new resource here I expect.
For questions these, I think you may benefit from some training or background reading of these classes and FHIR in general. It may not be easy to get the “big picture” by asking individual questions on here
Also I would recommend https://chat.fhir.org/#narrow/stream/implementers for general FHIR implementation questions, that are not specific to the UK. I am not sure if your context is UK.