Counter-question: does it really sound natural to you to use whom in your second example? Of course, if you do, then fair enough, but I suspect that:
- most speakers would automatically use who and find it more natural;
- if anything, whom split from the preposition is something of a hypercorrection-- it may well be that what makes the stranding preposition so productive is partly the breakdown of the case system, so there doesn't seem to be so much rationale in pretending that this is an example of "normal" case marking and maintaining "whom".
As far as I can observe, whom is only really very common directly after a preposition, and even then, who is possible ("these are the people for who(m) it's difficult to find work").
The prescriptive answer would generally be that whom is used specifically in cases deemed to be in the "objective" case. So this means direct and indirect objects, objects of prepositions, but not subjects or complements that aren't actually direct or indirect objects (so: "He was not the person who he seemed/appeared/was deemed to be"). In your third example, who would indeed be deemed to be the subject of the relative clause. The fact that this distinction doesn't generally come naturally to speakers shows that it isn't really case marking, and hence there's actually not that much rationale for using whom at all.
If you want to make your life simpler, you could just not use whom at all ever...