Here's my two cents:
1) Sure, it's worth exploring Auto mode. Set the min pressure high enough where you don't feel like you're suffocating when you're trying to get to sleep. Given that you've been using straight 15cm for several years, it's very reasonable to leave the max pressure at 20 to see where the machine wants to take the pressure.
2) If you wind up not liking Auto mode because the varying pressure tends to wake you up, you can always switch back to straight CPAP once you have a couple of weeks worth of APAP data. The beauty of owning an APAP is that it can be run in either mode. And you can use the APAP data to figure out a good starting value for straight CPAP if you decide to switch back to straight CPAP.
3) Since you've been sleeping comfortably for years and years with your old REMstar while NOT using Flex, I'd say turn EPR OFF. My guess is that since you might just find the pressure changes caused by EPR to be annoying and distracting since you are used to a fixed pressure blowing at you throughout the entire inhale/exhale breath cycle. And annoying and distracting things make it harder to get to sleep.
4) You say you're looking forward to trying a heated hose. Heated hoses presume that you are using the heated humidifier. It's well worth trying, but if you soon feel like you're trying to get to sleep in a swamp, then consider going back to a regular unheated hose and turning the humidifier down or off.
See my Guide to SleepyHead