I got an income affidavit from my embassy to meet the retirement visa/extension of stay, but Thai immigration said to come back in 5 weeks. I know I should've, but I forgot at the time to ask; can I use this same document in 5 weeks or does it need to be a closer to the time I get the extension? In 5 weeks I can show them a latest salary slip to corroborate the document. Also, if this document won't work can I use a joint Thai bank account-- with 800k in it, I have with my Thai wife instead of the affidavit? Thanks in advance for any sensible answers.

Your income affidavit will be accepted. The rule is that immigration will accept them that are up to 6 months old.

Immigration will not normally accept a joint account. Some offices will if you have 1.6 million baht in the account.