If I understand it right I can set up the next accounting period and add new transactions as they come along, and still be able to do end of year transactions for the previous accounting period.
yes.
unless you set a lock date under Settings you can still edit any transactions of any accounting period while setting the Summary to show only the present accounting period.
No, you can enter future dated transactions at any time but you may see a message across the top of the Summary tab advising that there are transaction dated later then the set period dates.