The new year is in a different folder that the previous year. So you can do anything to that set of data and not affect the previous year's data.
1) If you delete the Entity with File | Delete Entity then that Entity will not be able to be accessed until you add it back to the list of entities.

2) As for starting the year over with Utilties | Start Year Over Again, yes, you can start the year over. You can zero the starting balances or change the calendar year or change the year for the new year or any of the other options for starting the year over again.

3) Depending on what you want to do, you can delete accounts, payees, move accounts in the chart of accounts, rename payees & accounts, etc. But these things can be done at anytime not only when you are starting a new year.