Are there any combination of letters that a namecoin address cannot start with?
Namecoin addresses, like Bitcoin addresses, are Base58 encodings of a public key hash. The encoded data starts with a version byte, and the value of this byte affects the first characters of the result.

Namecoin addresses specifically use a version byte of 52, which dictates a certain range of initials. The prefixes you listed are simply too high - they correspond to a version byte of 53 or even 54. Likewise, some prefixes starting with M are impossible because they correspond to a version byte of 49, 50 or 51. You could give a more precise "lowest" and "highest" addresses, in lexicographic ordering - the address must be between MvaNCeVyvP6ZXYFWGpKaDX9ujEQ418F7sm and NKuyBkoGdZZSLyPbJEetheRhMjezbtjJCf.

A full list of version bytes and first characters is available here, and the possible 2nd character ranges can be interpolated from it. For version bytes 0-2 and 144-255 the address length can be different from 34 so it makes the ranges less intuitive. But for 3-143 it's fairly straightforward: You have about 58^2 possibilities for first 2 characters to go between 141 version bytes, so each is only allocated a range of about 23 combinations. For Namecoin addresses that would be Mv to NK.