|As long as you can live with the look, the M should work for you just fine. ME covers have their recommended weight range, and often people can use them above and/or below those weights. It really often comes down to personal preference and baby's build (like if your babe is a chunk, covers may come up higher/fit differently than for someone with a long, lean baby). Baby will grow so quickly that soon those diapers and covers will look totally normal.
Yes, in theory the M covers should be enough to take care of your S Sandys use.
The M/L is the size that many people skip, but that is only available in the Airflow, not the Rikki (as far as I know). The M/L is definitely skipable, but depending in baby's build, sometimes it can be nice (like if M is getting too small but you feel like L is ginormous).