Hey, late to the party... and it was definitely a party. No expert, but I own a Gen 3 G26 and it is my choice for CC. I love mine, but mostly because I have a hard time with a larger handgun for conceal carry. It is my only Glock, and after owning it, I have really learned to like it. I carry mine in a Minotaur MTAC IWB holster, and sometimes OWB with a Flamingo (local to Fredericksburg).
My slide release is stiff, but it works. Plus the release is so small and I do not have large hands, so using it during a mag change is somewhat a PITA. I force the issue when practicing and do both methods. If it doesn't break in a little after a few hundred rounds, I would get the extender, or have someone look at it.
My G26 10 rnd mags release and will drop out fine. However, I noticed they were a little less likely to drop free when brand new. Mine has about 500 rnds through it though.
I have never used a 3 round pierce extender. I have 4 10 rnd mags, 1 with a 2 rnd pierce extender (really just the pinky extender, and only adds 1 rnd TBH), and haven't had a problem with it. I chose to go 10 round G26 for 10+1 when carrying, and then have a 15 round with a base cover on it so it matches the grip for my backup mag(s). I thought the Pierce would help accuracy, but TBH it doesn't for me, and I actually like the baby glock grip now, and accuracy is no better, or worse
BTW. Some of my (stock) mags will rattle also when fully loaded. I have looked into it a little, and supposedly it is the first round in the mag loosing tension on it or something to that effect. If you tap the base of the mag on something, it will settle it. Also, if you shoot it a bunch, it'll all break in.
Shoot a few hundred rounds through it. Then see how it does for you. I am ready for a 30s myself. Kind of hoping my wife will take a liking to 26.