Q1. It might be helpful if you listed all the possible post you have, with sources, dates, and whether it was addressed to the applicant, the sponsor, or both. Then we can see what you have to work with and look for the best solution. That July 18-September 18 pairing for individually-addressed items is two months apart, and that's pretty far. But, really, you can only send them what you have, so if that is the best you can do, it'll have to do.
Q2. I expect the pension company letter would be acceptable as it's akin to a bank, I believe.