Hi, sorry, it looks like your question fell under the radar. I’ve never done frozen (or other) corned beef in the IP. I have done pork shoulder for pulled pork many times.
To the extent the two might be analogous, the non-frozen instruction to go 70 minutes seems long to me, if you are using the high pressure setting on the IP. For 4 pounds of shoulder, I normally give it 50 minutes on pressure, let it cool about 20, then release the rest of the pressure. This is fork-shreddable tender. I once followed a recipe calling for 60 minutes and full time natural cool down and it ended up with a very dry texture.
The frozen aspect throws me for a curve, though, as I’ve never cooked meat from frozen. I’m hoping that my reply will jack your question back up, to where someone with more expertise might see it and respond.
Have you made it yet? If so, what was your total time, and how did it turn out?
