yes, to enable dual channel the memory modules usually have to be in slots 1 and 3 or 2 and 4, check your motherboard manual
Ah yes that could be the problem. I am sure it will be 1 and 3 no matter the board. Make sure you have the first DIMM in slot one (often the closest slot to the CPU), then the second DIMM in slot 3 (the next but one a long). I think that if you have the second DIMM in the second slot it wont be recognised?? Not sure on that tho.
Not necessarily, although this is probably the most common configuration. Plenty of boards don't do staggered slots at all. An old ECS NF4 board I had used side by side slots instead of every other, and so does my current board, a Gigiabyte MA-770-DS3P.