1. in short, no, there is not a big difference. in fact, i prefer the used key switch is it is not as stiff as a new one. some say the stiffness offers a little more tactility but i personally don't notice it. you are fine.
2. i just prefer my keyboards to be new.
good question on finding NIB mx-11800. unless you find a solution, just patiently use your used one and wait for a deal. when that happens then nab it and put your used kb for sale at an opportunistic time.