Help validating our proposed WordPress/CiviCRM solution!
In general I would say that this sounds feasible.  Your requirements sound similar to how many other charities are using CiviCRM.  However, you have asked several detailed questions and I think you could benefit from discussing this in more detail with one of the CiviCRM partners (and/or posting these as separate questions).  

I would also recommend that you attend a CiviCamp to find out more about how CiviCRM works and meet others in the community.  We are a friendly bunch and you will get a lot out of meeting people face-to-face.  I see that you are based in the UK and can highly recommend attending CiviCamp Manchester on 6 October 2018 if you are able to.