For clarity Qlik Cloud is a separate product currently in beta. I believe when you say cloud in you question you are referring to a Qlik Sense instance access via a browser. Here are the answers to your questions:
1. Qlik Sense is fully online so the developer would access Qlik Sense via a browser and connect to the server. It would be technically possible for the developer to use Qlik Sense Desktop however Qlik Sense Desktop is intended for personal use not as a development environment for the full Qlik Sense product thus it wouldn't be a smooth workflow.
2. Yes QlikView apps (QVWs) can be converted to Qlik Sense apps (QVFs). The load script and the data model will convert over instantly. Since the two products have very different interfaces and purposes the visulzaitons do not convert over. You may which to review Qlik Sense Desktop - Converting a QlikView Data Model (video)
3. Yes external users could access Qlik Sense if this is desired (it can also be prevented if this is the desired behaviour). Any users access Qlik Sense will need to be licensed. The licensing is based on consumption or amount of usage not type of usage (it is not a 'viewer', 'power users' etc. licensing model). Our sales team can help you understand the cost once they understand the usage behaviour.
4. Qlik Sense and QlikView are similar in some respects but very different in other respects. So no they don't do exactly the same thing but there is overlap.
5. Our sales team can assist you with pricing. Would you like me to have someone contact you?
I hope you found this helpful.