… having multiple issues with High Sierra (i. e. when inputting notes in Speedy Entry...it will only input rests using the numbers keys;  I can't change the page format to landscape) …

You tell that you have “multiple issues”, and you mention two of them.

- What are the other issues, I mean the issues other than the two you mention?

About the two issues you mention:

1) … when inputting notes in Speedy Entry...it will only input rests using the numbers keys …

I am not sure I understand.

Are you asking for another way to input rests - without using the number keys?

Or is the issue that you need to input notes, and get rests instead of notes?

- Are you using a MIDI device for input?

If you are using a MIDI device for input, go to the Speedy menu, and select the menu item “Use MIDI Device for Input”.

2) … I can't change the page format to landscape …

Please specify the steps so that we can try to reproduce the problem.

- What are you trying to achieve?

- How are you trying to do it?

- What happens when you do that?

- What doesn't happen that you expect to happen?