Currency exchange questions

Questions for changing money: 1. Are the forex bureaus in Bole international Airport open 24 hours (plane arrives at 6am)? 2.How are the rates at the airport compared with those at the banks and forex bureaus in town? 3. Do forex bureaus issue receipts? Will they allow changing excess birr back to USD by showing the receipt? 4.Is there a zone in Addis Ababa where I can find a few forex bureaus close to each other (for comparing rates)? 5.Do the regional airports also have exchange offices (Dire Dawa, Gondar, Lalibela, Mekele) or will I find exchange offices in these towns? I suppose exchange money in banks would take a long time. 6.What is the maximum withdrawal limit in birr at ATMs? Thanks in advance for any advice.