@lisa
I believe the reason you’re unable to properly edit APNs is that the carrier can control the values, rather than a bug in the Mudita software. I was having a similar experience with AT&T (though the way it manifested was slightly different, I could enter a new APN but it would disappear after saving).
Because of this control the carriers have over the APNs, I don’t think you’ll be able to fix it this way. The root cause is that the Mudita Kompakt has not been registered with Verizon/AT&T. (I just posted my whole saga here: Mudita Kompakt does not work with AT&T)