For reasons I'm not entirely sure of, the <readmore> text in my "preview" does not show any signs of "different" formatting

I logged out and previewed a post containing the following:

foo
<readmore>
bar
</readmore>
baz
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"bar" had a (very) light grey background. You have changed to a theme that doesn't do that, your personal CSS overrides it, or your monitor's calibration is off.

I was simply meaning that it would be nice if the "preview" showed exactly what would appear in the post.

Again, it already does that, ...

e.g, <readmore> tags would be collapsed,

...readmore sections are expanded (not collapsed) when you view a post with readmore tags. Readmore secions are collapsed when embedding the post in another post.

Now if I could just get my "preview" to show such text as "different".

I think I mentioned you could change your personal CSS. Specifically, do something with the "readmore" CSS class.

.readmore {
    border: solid red 0.5em;
}
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