Not sure I can answer all your questions, but as for the question #2 and #3, I believe I’ve an idea.
I don’t see that Elk has a product, but there are a number of wireless relay units out there. Ranging in a number of costs, but this cheap one is on Amazon – https://www.amazon.com/3-7-12V-Wireless-Control-Transmitter-Controller/dp/B08F7FFQ14/
Perhaps the answer for #3 could be the addition of a another wireless relay as in question #2. You’d have it wired up on Output 2 on your Elk M1 Gold as voltage and have it trigger a local relay that is 2 outputs, one drives the local siren and the other is the transmitter to your remote location. That relay would then connect a siren mounted out in the garage. If the alarm is triggered, you’ll have the siren sounding there as well as in the primary location.