1. I believe it is 28 days.
2. Combining postage has not been possible for ages. Buyers have to pay in full and seller refunds excess amount. This however leaves you if you combine with only one tracking number. I believe there was something in a recent announcement that now allows tracking to be marked on each item.
3. If accepted at Post Office should be OK, Royal Mail recently had a problem, now sorted so they say, so possibly PO may be accepting even wrong labels rather than sending sellers away.
I must admit what I know about Simple post is only what I have read, and ebay keeps making changes. I do not use SD and never will, as it is not Simple and very inconvenient to use. I do not want frequent visits to PO and collections would not be practical.