Will a client running version 13.5 be able to open a solution created in v13.9 running on FM Server 13.9?
The version the solution was created in doesn't play a role in your scenario. It's whether you turn on SSL on FMS that will determine wither v5 can connect to v9.
It depends. Take a look at this page: Security Update - FileMaker 13.0v9/FileMaker 12 | FileMaker
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