1. No - you don't need to enable period end JEs for importing historical opening balances. You can load them directly into whatever first open period you want.
2. Yes, you can map the historical account to the external ID, but I wouldn't recommend it. For ease of imports going forward, I tend to map the current account number to the external ID. I'd recommend creating a separate custom field for the historical account number and map it there if you need it for reconciliation/reporting purposes.
3. Statistical JEs are for tracking non-financial numbers (e.g. headcount or square footage for allocation purposes). You definitely should NOT be using Statistical JEs for loading opening balances. You can load opening balances when creating accounts, but I almost always load historical balances as a separate journal entry, using the standard Journal Entry record. IME, it's easier, safer, more transparent, and less difficult to change/adjust if needed than loading opening balances directly to the accounts.