1.) Yes, it's a Gen 2 Sigma.
All of the SW# Models are part of the SIGMA Family, as are the current SD#VE pistols by extension. Some folks will argue over this, but only because the SIGMA has a bit of a stigma against it because the Gen 1 models had issues. Said issues have long since been addressed by subsequent revisions, but the stigma remains, so certain folks like to pretend that the SD Series isn't related to the SIGMA Series. However, Smith & Wesson Customer Service Reps have let it slip that at one point SD stood for "SIGMA DELTA" or SIGMA Gen 4, but decided it was better to market the SD as an acronym for "Self Defense" instead due to the persistent negative reputation of the SIGMA Series.
Basically, it works like this...
SW#/Micro = Gen 1
SW#Enhanced/Value = Gen 2
SW#Green/ValueEnhanced = Gen 3
SD#/VE = Gen 4
2.) Yes, all SIGMA Magazines are interchangeable.
Gen 1 SIGMA Magazines will require some minor modifications or replacement of the thicker polymer baseplate to fit into the magwell of newer SIGMA Models, however.
3.) Yes, aftermarket spring kits are offered by a variety of manufacturers.
However, they're not all interchangeable, so make sure that you get one which is marked compatible with your specific model. I believe that Gen 2 and Gen 3 SIGMAs all use the same springs, but the Gen 4 SD models have a different trigger arrangement.
That being said, I recommend leaving the trigger stock unless it's broken, because unfortunately the SIGMA trigger is what it is, and attempts to lighten the trigger often come at the cost of reliability. If all you shoot are lighter loads then it might be fine, but stick full power 180gr Self-Defense/Duty ammo in there with trigger mods and the sear may fail to reset.