Hello again.
I got a chance to look into this just now, and I am baffled as to why it suddenly doesn't work anymore. I enlisted the aid of the VS team, as it's obvious something they changed with how damage popups display in the last couple of version is causing this problem, as it works without BattleCore, and it worked on at least v1.00, honestly I hadn't tested on anything later than that, and I can't seem to access any files between 1.00 and 1.04.
I tried several things to find where the crash actually happens, or what my plugin specifically is doing to interfere with the popup in BattleCore, but everything points towards an issue in BattleCore itself causing this, as as you know, that plugin is obfuscated.
Yanfly advised that he made a change in an upcoming patch which will come on out Sunday, that may resolve this, or at least stop it from crashing entirely so I can troubleshoot it better. So I will leave it with him for and follow up when that update is out to make sure it's resolved.
As for your other suggestion about the showing of BLK and SHDPRC on the equip scene via the CoreEngine plugin parameters:
I managed to get BLK and SHDPRC to show up correctly on the default equip scene in ItemsEquipsCore, meaning when you have the 'use updated layout' parameter set to false, but it doesn't seem to show on the updated scene. Also, my change apparently causes a crash in the item scene when viewing any item at all, whether the updated scene option is used or not. (This turned out to be an issue with updated plugins having blank parameters, whoops)
I will continue working on this and get back to you.