These sentences are grammatical, and I would say they are easy to understand... eventually. The meaning itself is relatively complicated, and you might have to read it slowly a couple of times, but I think it's been carefully written so that you will get the right meaning in the end.
It could be a little less stilted and formal:
You will get your pass the month before the course starts, but not
before then. This letter will expire on the 12th of October.
(since the relevant date is printed on the letter, they shouldn't be requiring you to do the math, but that's an admin issue rather than a language issue).
UPDATE: to answer the second part of the OP's question, the meaning of the sentences is:
- The pass will be issued one month before the commencement date, or after that time.
- This letter will expire when the course starts, but if the course starts in less than two months, this letter will expire two months from now.