Looking for a word that describes this emotion: I'm amused, but ashamed to be amused at the same time

Schadenfreude fits most of your examples. The fact that English had to borrow a German term to cover this would seem to indicate that there is no native word that covers your actual answer.

Sounds a bit like chagrinned. American Heritage defines chagrin as

A keen feeling of mental unease, as of annoyance or embarrassment, caused by failure, disappointment, or a disconcerting event: To her chagrin, the party ended just as she arrived.

While it is not limited to embarrassment associated with amusement, it might suit.

There is also the concept of guilty pleasure

Something, such as a movie, television program, or piece of music, that one enjoys despite feeling that it is not generally held in high regard

Oxford Dictionary Online

The idea is I like it but I shouldn't is a good bit broader, but it aligns with highbrow types giggling at lowbrow culture.

I think bemused might be a good fit, though I'm not sure if the connotation is identical. You could check out some real life examples to see if it works for you.

P.S. Check out this brief article - We are not bemused

I suspect bemused might not work for you as a standalone term. For example, people might not know exactly what the following means:

I was bemused.

But I think they could easily fill in the blanks by focusing on context, though I'm having trouble coming up with a good example. Below is one possibility, though I'm not 100% certain that I'm using the term correctly:

People who are amused by handicapped people are sickos, though I have to confess to occasional bemusement.