Does diagonal or vertical position affect the lifespan of a SSD drive?

SSDs have no moving parts at all. As long as your connectors don't experience forces they weren't designed to take (wrenching them out of their sockets), you should be fine. They can get damaged, but thats due to catastrophic damage. Their time to failure due to old age shouldn't be affected much by orientation. With a case that crammed I'd be worried about airflow and temperatures, but that would affect your entire system, not just your SSD, and be independent of orientation.

The mounting angle of a SSD should not have any influence on its lifespan.

(Assuming it does not slip down and blocks a fan, which would be bad for other components in your case)