The DUTY is the older (first?) version. I have one in .40. It is still my EDC concealed pistol after 10 or 11 years of carry/shooting.
The 9MM did have a 16 round magazine at first. I've had no issues with mine (I got a DUTY 9MM slide assembly and magazine for my P07 a few weeks after I bought it.) Later I got a P07 9MM (the newer model) and I'm not sure where the 16 round magazine ended up. It's no longer in the box with the older DUTY slide assembly. Probably in a magazine pouch for the 9MM P07.
As to magazines not locking back the slide. Do this:
Make sure the magazine and chamber are empty and insert the magazine into the pistol. Pull the slide back and watch that tab on the slide lock to see if the empty magazine follower pushes the slide stop up to engage the notch in the slide and lock the slide to the rear.
If not, remove the slide, reinsert the slide lock into the frame and take a good lock at how the follower in the empty magazine is interacting with the slide stop. It should make good contact and push the slide stop up to engage the slide.
If it's not doing that you might want to buy a new magazine spring for it. You can get slightly stronger magazine springs for the P07 magazines to help insure the follower (when the magazine goes empty) does contact the slide stop and push it upwards fully.
Every not and then you'll run across a magazine follower and slide stop that just don't seem to be made for each other. Not sure if the follower is made incorrectly, or worn/damaged, or the slide stop has been modified. Some slide stops will actually stick in towards the center of the pistol far enough to lock the slide back when there are rounds in the magazine because the bullets make contact with the slide stop and push it up (locking the slide back).
I think both of these slide lock/release levers are from P07's. One is for the .40 and one for the 9MM (one could be for the P09 9MM which would be similar to the P07 9MM). Don't know if CZ still makes two different slide release levers or not (probably not as there are no more .40 S&W P07 or P09 pistols being made. Also could be one is the older DUTY model slide release and one is the newer model version of it.
Does the mag. release lever in the DUTY pistol you might buy look like the more angled one in that picture or the straighter one?
I ask because it could be the follower isn't make good contact with your slide release lever due to the design of the slide release, or because the mag. spring is weak.
If you buy the DUTY pistol be aware that the newer P07 and P09 magazines will still work it it.
Not sure if the very latest dual pistol platform mags work in the older DUTY P07s or not. The newest version of the 9MM magazines are cut to work with P07s/P09s and the newer striker fired P10 models. It may still work (I would think it would, but can't guarantee it as I haven't bought any of them to try).
My P07 DUTY has been trouble free and is more than accurate enough to suit me and I'm pretty picky.
Lots of folks here on the forum to help you with any issues or questions you have if you buy either pistol and want to upgrade it to make it smoother to use/shoot.
Good luck with it.