The cabin filter in the gen 1 is VERY easy to replace yourself. No tools required! The gen 2 on the other hand is a nightmare from what I've read. It requires a socket wrench and an extender to reach the bolts that secure the hatch cover that holds the filter hostage. If you have the tools and the patience, you can do it yourself, otherwise it might be less hassle to let the dealer do it. Entirely up to you though.
I've replaced my Gen 2's filter. It honestly wasn't that difficult; took maybe 20 minutes. You are correct in saying that you need a socket extender, but my dad had plenty of tools and once I got all the bolts off, it was a matter of pulling the glove compartment out, swapping filters (replacement cost me $17), and snapping the glove compartment back in. Could have been easier, but it wasn't intensive work in the slightest. The glove compartment is lightweight.Got it. Just wanted to make sure.
I've had my Volt since Jan 2016 and it still smells brand new inside. Just recently though when the AC is on and recirculating there's a weird filter-e smell. Basically that smell when you have the re-circulation off, it smells like that, but with it on.
So hopefully replacing the filter does the trick
For some reason, I still have an odor in mine even after replacing the filter. It's only occasionally, and it only started happening after my car sat in my dealer's shop for 2.5 weeks waiting for a HV Battery Section 3 replacement to come in. It can only be described as a musty organic smell, almost like meat that's gone bad, but nowhere near as pungent.
So, your mileage may vary with a filter replacement. Mine is so infrequent, I'm not really planning on trying to get it fixed unless it gets worse.