In simple terms multi channel memory is a way to save memory write cycles. Each channel is treated as a memory bank and when the opportunity arises the same data can be written simultaneously to all of them, rather than having a cycle for each. In theory it would be 4 times faster, in practice this doesn't always happen, but it is an effective way to increase bandwidth.
Lower CAS latency (CL) is how quickly the memory can respond to commands. The lower it is the better, and means that data gets to the CPU that much faster. Not sure if I am 100% remembering correctly, but I believe that equates to clock cycles before the data in the memory will change after a command is issued. So at 3000Mhz (effective) that would make 1500Mhz at CAS 15, equates to a nice round 15 nanoseconds to respond. If you start adding up 15ns vs 16ns over time, you will end up with quite the difference. So it is a big deal in theory. In practice, it just makes the performance a little better.
It is recommended to buy memory in kits. They are binned and matched together for their rating. There is no guarantee two sets, even of the same model, will perform properly together.
I don't really see the need for 32GB of memory unless you do something other than gaming that requires that much. Some of the heavier games out now need about 11-12GB of memory. That is a system total. Having another 20GB on top of that will just sit idle under most circumstances. Unless you like to keep lots of programs running all the time.