Let's not get into sweep or we'll be here all day talking about thickening wing-tip b-layers/tip stall/pitch-up etc!
Wilfred, cannot answer the question about 0.61M - it depends on how much the air accelerates over a particular thin or thick section. It only requires LOCALLY sonic conditions to cause separation. Mach 1 is always REACHED somewhere in the airflow around an aircraft when the actual physical Mach is below 1.0. Pulling 'g' aggravates the compressibilty effect. Ask any FJ pilot who has dropped a boom in combat while 'subsonic'! Try the B737 at 0.8 Mach and a steep turn (in the sim, of course!) - you'll get shock buffet.