Welcome to the forums.
1. Any experience rating of good/bad, is a matter of personal opinion. I myself, love the Android experience. I love being able to setup my phone the way I want it. It will not look like anyone else's. One of my main issues with IOS, is that all the phones look exactly the same. Same number of rows, same number of columns No individuality.
2. I think since 4.0 (Ice cream sandwich), all apps have access to all internal memory. The memory used to be separated between operating system and apps. In the earlier versions of Android, when you had limited app space, it was up to the app developer whether or not you could move a app to the SD card. Even when you could, the entire app was not moved. There was always some part left on your device.
3. Most of the time, the minimal requirements are based around the version of Android that was available when the app was created. Older apps will likely have lower version requirements than newer apps.
3b. As Android versions increase, the amount of things that can be accomplished with apps increases as well. Sometimes a app will be created that relies on something that the lower versions of Android do not have or allow.
Hope this answers your questions for you.
Again, welcome the forum. Enjoy your stay.