Regarding your questions about completion behavior:

You don't have to scroll, you can continue typing and the completion list will filter itself to options that match the additional characters. Deleting characters works too (options reappear in the list).

The second thing you mention is having to manually type the parenthesis after completing a function/method name. A long time ago, I had a prototype which added them automatically, but I never got it polished enough to add to IDLE. However, this has rarely been raised as a request. Also, Terry said he wants to refactor the code and get it well tested, so I imagine new features like this won't get in any time soon...