Hi all

Well I'm not a complete newbie to this having done a fair bit of research over the last few months, but I do have a few things that would be great if I could get cleared up re UK visas!
My boyfriend is American and I am a British citizen. We fell in love about 18 months ago when he was studying in the UK and have seen each other twice since with plans to see each other this spring also. We want to be together properly and live in the same country and to this end plan to enter into a civil partnership later this year so that he has the legal right to live and work in the UK. We want to enter the civil partnership to formalise our relationship and commit to each other also.
As I understand the process we would have to first apply for a Fiance Visa after which he could come to the UK and we would have six months in which we would enter the civil partnership. Then we would have to apply for a Spouse Visa so that he could live and work in the UK for 30 months at which point we could apply to extend the visa for another 30 months so that he could get Indefinite Leave to Remain status.
My question is, are the Fiance Visa and Spouse Visa processes completely separate, with all the associated costs, or are they in some way bundled into a single application as would seem sensible? I can't find the answer to this online!
Also, what is the likelihood of requiring an interview to secure either the Fiance or Spouse Visa? Our relationship is very much genuine and this is easily provable. What sort of evidence is required to satisfy the Home Office of this?
Finally, because we are both relentlessly pragmatic and want to cover all eventualities no matter how unlikely we consider them, if we were to separate before we had been together five years what would be the implications for his right to live and work in the UK? Would he be asked to leave? For context he is about to graduate with a good Data Science degree and will work in the tech industry, probably earning a pretty good salary - does this affect the situation?
Thanks for your help in advance!
Alex