Question #1: The SR3100 instructions say it uses "DSM2". Will the SR3100 work with the first release DSM module I have for my EX10?
Like Dave said, yes it will
Question #2: Can the antenna lengths on the SR3100, SR3100Pro and SR3500 be cut shorter than factory default? If so, how short (I did not see this on the online manuals) for each respectively?
Why? Just cut a shorter antenna tube
Question #3: Does the current SM1001 DSM Pro Series modules work with all SR#### (3000, 3100, 3100Pro and 3500) series receivers? If not, which?
yes