Pluraj is simply "more than one." Kelkaj is ne tre multaj. Multaj is "a large number of." So, strictly speaking, pluraj coud be more than multaj or fewer than kelkaj, even thought multaj is more than kelkaj.
So... I wouldn't expect these to map perfectly onto English words regardless of what Merriam-Webster says.
Salivanto is right. If you can connect to the on-line station "Muzaiko", Then listen carefully during the interviews. Folks will often trill their R's in ways to which most English speakers are unaccustomed.
Besides, it's good practice for hearing the language and there's some decent music there.
In fact, you are totally right here. The preferred form is "estas klaraj". My understanding is that there is some internal debate on the Duolingo team (nothing wrong with that) and that the apparent preference that the course has for using adjectives as verbs ("klaras" instead of "estas klaraj") is due to the personal preference of a few individuals. There is nothing wrong with saying "klaras" in the right context, but it is my opinion that putting such an emphasis on this form at this stage in learning gives the wrong impression to new learners. In the exter-dualingo world, it's preferable to say "estas klaraj" here.
Mi adas aŭdi "pluraj jeliguroj"--ĉu alio ajna spertas ankaŭ tiun ĉi?
I don't know if this sentence has audios from different speakers; listening to the ”original” male voice (the one with studio conditions, no reverberation) I clearly hear ”reguloj,” with a tiny little shwa between ”pluraj” and ”reguloj” (”pluraj-ə-reguloj”). The alternative would be a clear break between the two words, which might sound unnatural. I agree that the ”ə” may be a bit longer than necessary.
(”Does anybody else…” translates, to be precise, to ”ĉu iu alia…”)