This image is the differences of phonetic symbols between IPA (left), AHD (middle) and Merriam Webster (right):
The two Merriam Webster phonetic symbols are the same, so how am I to know the accurate pronunciation when it appears in a new word?
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To answer your last question first: Use a dictionary that follows a widely-accepted standard; that's what IPA is for. If you really want to use Merriam-Webster's pronunciation guidance, you'll need to refer to their own notes.
Going back to the source of your table: I wouldn't trust it. There's a lot wrong with that page, which appears to be just a personal opinion. In particular the author has ignored the distinction between \ə\ and \ər\ in the M-W guide.
I think the examples are the author's own, and I don't think the author is a native speaker of a common dialect of English. Some examples only make sense in a rhotic accent (seemingly treated as default by Merriam-Webster), while others don't make sense in the same accent.
M-W has \ˈwərk. We can't neglect the r in that transcription as the author of the page you linked does (see p.2 of the pronunciation notes I linked in my first paragraph).
As I commented, courage doesn't belong in the first list, in (most) English or many American accents: