Request for information on China Visa.

Im a Nigerian and i need it to China, i have a letter of invitation from my business partner in China but it is not confirmed by Ministry of Foreign Affairs in China, is it compulsory it is confirmed by the Ministry?

Also i was told by a friend that the letter neneds to be up to 6 weeks with me before i take it to the Embassy or else the Embassy will not honor it, are these points true?

Please advise, Thanks.