Protection and hidden sheets

1. How does a user navigate between sheets with keystrokes (on accident in this case)? Is there a way to prevent this?

With non-hidden sheets, you can move between them using CTRL+PAGEDOWN or PAGEUP. I'm not aware of how to accidentally land on a hidden sheet using keystrokes. If you want hidden sheets to stay hidden, you can hide them and then protect the workbook/structure. You don't need to protect the individual worksheets if your goal is just to keep sheets hidden. (The other way would be to use vba to make the sheets you want to hide "xlVeryHidden".)

2. Is there a way to protect the preferences on the worksheet so they can not be changed?

You can protect worksheets using the Tools menu (Protection-->Protect Sheets). This will stop users from changing formats and any locked cells. If you want users to be able to edit parts of the worksheets while protected, make sure you unlock them first. (Format--Cells...--Protection --Uncheck 'Locked')