Q1. You'll need survival-level skills with linux, imo. If that's new, consider setting up a virtual machine with VirtualBox and playing with Ubuntu. You should be familiar with Ethereum from a user perspective, so the wallet and MIST which you will later learn to use to talk to your smart contracts. Find some existing contracts (tutorials?) to talk to with MIST. Thinking not as a developer but as a pro-user, install the geth command-line tool and learn how to create an account and start mining. You won't make any money but it will familiarize you with the surface area of the beast you'll learn to control.
Q2: Decentralized apps mean the important data and logic is not located on a specific server and anyone with the UI will be able to use it. Actual delivery of the UI means shipping the files, like index.html to the user and in many cases devs use traditional domain/server or cloud content-distribution networks to accomplish it. Not entirely decentralized.
Q3: The blockchain doesn't consider that anyone is any better than anyone else, but contracts can enforce logic such as access-control lists ... only do this if the transaction is signed by such-and-such an address who is the sysAdmin (owner). You can change the UI any time you want, but you only get upgradable contracts when you use advanced patterns and modular design. The default is your code cannot be changed. Any revision is a new initialized instance.
Q4: No. The flows are too different and other re-orientation is needed. It is not a 1:1 replacement for either server-side logic or storage. It's something entirely different. You would not be able, for example, to convert Basic or PHP solutions to Solidity on a translation basis. It takes some time to internalize what this platform can and cannot do and to acquire a good sense of how to tackle various problems. A server can indeed be a client to a blockchain app much it can be a client to an API. An API-first approach often helps, as well as a very minimalist approach to what the contract should be concerned with. I've observed that you'll find a lot of traditionally server-side concerns migrating to client-side, leveraging much smarter clients.
Hope it helps.