That should have no problem fitting, even though some are the rounded connections, they still fit in a square port.
The side of the EPS cable with the clips should go to the right in the second picture, matching them up looks like it should be fine.
No. That 4+4pin is what's used. However, you must actually fit the 2x halfs together, they are keyed with slots, to make a single 8pin. If you try them seperately, they don't fit. The snap tab goes on the inside of the mobo, not towards the outside.