So I am and was in very similar positions.
I have an Alien Gear Cloak Tuck for my Glock 23. I had ordered it but it took nearly 8 weeks to come. During that time I had realized I needed an every day carry pistol dedicated for that task and started my search and research etc. and decided on the Shield. The Alien Gear came in, and I tried it out for a week w/ the Glock 23 but decided it was the smart thing to do to buy the Shield and commit to it for EDC. I spent all I had on it and ammo to break it in and had to wait a month to buy accessories including a proper IWB holster.
While I appreciated the Cloak Tuck holster it really bothered me they made no mention of any wait when I bought it; that is, until I got a confirmation email saying they charged my card, they then said , "There will be a 4-6 week wait" which was nearly 8 weeks. Plus I had several contacts with them during the wait and it was all brush-offs and Who Mes? Oops! We'll try to do better next time! Better luck next year! So I went with a Galco King Tuk for my Shield.
I have been carrying it in the King Tuk for about 1.5 months now. I could never get comfortable with it at 4'clock (I am a small guy w/ a small frame etc.) and therefore I moved it to 5, 5'30, nearly 6 oclock and it was much more comfortable but I always felt it wasn't right. ALSO- To your point, I frequently would get out of the car after driving or out of a chair etc. and realize my magazine had come unseated. Lastly, I had some trouble initially trying to alter the ride hide and cant... one side had no problem but the other size, one screw would NOT get unscrewed. Everytime I tried , the gold washer thing would just spin and spin. I ruined the screws and gouged the leather trying to I just gave up. I lowered the ride height but increased the cant by leaving one side normal and adjusting the other.
Last week I decided this is it, I am going to get this freaking thing right. I basically used needle nose pliers to grab the washer, gashing the leather a nice bit but getting it done. I finally lowered the ride height the way I wanted it but while putting it on, I accidentally moved it to the proper 4-4:30 position. And I said, let me try this the way they said its supposed to be. And actually it worked!
Now I still feel like the butt of the pistol sticks out a bit but it seems to conform to my back like its supposed to. PLUS, what really makes a difference is the gun is on my hip now, not my back. So it carries, and feels, a lot better. The weight is more distributed properly and the gun seems to stay glued to my body whereas when it was near the center of my back, it was more like it was along for the ride. Furthermore I have not had a release-of-mag yet.
I do think I will need another month or two to get used to this since the last (and first) 1.5 months, it has been slowly conforming to a different part of my body. But I believe in the long run, it will be a lot better.
I read initially someone say about hybrid Leather/Kydex IWB holsters (Like the Alien Gear and the King Tuk) that it's all about finding where you need to put the clips, when you find the proper position of the holster it's all gravy from there. I think it's pretty accurate.