I've got an older 183-series Mini that I bought new back in the early 80's that will happily digest any ammo I decide to use. 5.56 NATO or .223 Rem. it doesn't seem to matter. One of the well known features of the Mini is its reliability. I can't say that I've ever known any of the many people I know who own a Mini that have ever experienced FTF's or FTE like you've encountered. Clearly there was something uniquely wrong with that one. It happens sometimes with any manufactured product, but it doesn't mean they will all perform like that.
Hopefully the new one Ruger is sending to you will perform up to your expectations. Keep us informed of your progress on this matter.