Hi jki - ok so it is those slivery bits that is messing things up. There is a whole chain of them here (and on the other side of the model as well):



I've attached a 3DM file with those slivers removed and this version should now boolean ok with no ShrinkTrimmedSrf used.

I removed those slivers by selecting the larger neighboring faces and using Edit > Separate to break them off the main solid, then use Edit > Hide to hide them temporarily, then it's possible to select the slivers and delete them, then unhide the big faces using Edit > Hide again, then use Edit > Join to glue it back together.

The problem with degenerate faces is that the opposite sides of the trimming boundary touch each other and that makes the outer skin of the model essentially have self-intersecting areas and anything that tries to go over that degenerate surface will lose the proper sense of orientation.

- Michael