Said memo was issued after the pitch document I mentioned before, which is dated March 11, 1964 and already includes the entire cast I listed above. Therefore, it does not reveal anything whatsoever about the order in which the characters were created. I think everyone knows that Number One was written for Majel, but that tells us nothing about when the character was created relative to the others, or when Spock was created.
I may have to concede this point to you, I have looked and relooked for the source for where I read Number One and Spock were the first two characters conceived of for Star Trek and have not been able to find it. I may have at some point mistaken first cast for first conceived. If I do come across the source I will post it asap.