Quad channel DIMMs are picked to work in a 4 channel configuration while dual channel DIMMs are picked to work in a 2 channel configuration. They are mechanically and electrically identical and perfectly compatible, it's just a matter of ensuring that they've been tested to operate as marketed.
No lol, you have full access and timings/latency do not change. How many channels refers to how wide the memory interface is. Basically, it means that you have half of the memory bandwidth that you would have in a quad channel system (IE LGA 2011 with an X79 motherboard). This is no big deal because it won't harm performance much in most software. Compression/decompression, rendering, AVX accelerated floating point work, and some types of folding might see a large hit in performance compared to quad channel, but most software doesn't make much use of any of this and where it does, not significantly. If you extensively do any of the above, then you might be better off with a more high end computer.