I am sorry, I unintentionally forgot to type a correct title for this question:
Cannot mirror and get a solid body from a thickened surface.
You 've hit on a long standing problem with SW and the reason that I don't use thicken in these cases. The problem is that when the surface is offset to a solid, the side faces (walls) are created and their direction is derived from the normal of the surface. Because of tolerance, these do not necessarily equate to being co-linear with the planes (side and top Ref planes) and there is an almost imperceptible angle between these end faces and the Default side and top planes; this is why a bolean error occurs when you try to mirror them to each other because a very small sliver would try to be connecting them and SW throws a "fail to merge bodies" error eventhoug theoretically they should merge and you did constructed every thing correctly. This is evident by the fact that if you take a look at the attached part that I amended from yours, you'll see that I corrected this very slight intolerance (angle) by using replace face. After doing that everything works fine with mirror.
Your alternative method is what I usually do and would have done in your case.
MirrorProblem-mab.SLDPRT.zip 674.9 KB
Thanks! Using the replace face and delete+patch I could fix the parts I am working with.