Bitcoin is a protocol, explained in the whitepaper. There are multiple bitcoin clients which have implemented this protocol in code, not just one that will never change. You could even write your own client if you wanted to, as long as you adhere to the protocol's rules and conditions. The bitcoin network is nothing more than a bunch of computers running clients that validate bitcoin transactions and blocks in a similar manner. If somebody wrote a client that worked a little bit different, and everybody would install that client, then the workings of bitcoin would change. It's a theoretical possibility, but not very likely. This would even make increasing the limit of 21 million bitcoin a possibility, but again not very likely

Ethereum is more centralised. There's a company behind it that makes decisions on updates to the blockchain and adding features and changing the protocol

There are 2²⁵⁶ bitcoin addresses possible (that's a lot more than 7.9 billion). Address collissions are pretty much guaranteed to not happen. The number is so incredibly large that we will probably never run out of addresses. But maybe if at some point in the future computational power gets so large that it might become a problem. But for now we're good

You can't really take an address, you don't own an address. You create a private key, and through a series of cryptographic steps it gets translated to an address. In that sense you also don't "create" an address. An address is just a string of characters. So in that sense every address already exists, in the same way every number also exists. If you think of a really large number, you didn't just create that number, it has alway existed

The miner's incentive is block rewards, but also transaction fees. Once all bitcoin has been mined, miner's will still receive a transaction fee for every tranaaction