"slow down a bit" works better, as the English translation comes off as more direct than the Chinese.
It sounds good to me, too, but Duo doesn't accept it. However, the course isn't stable yet, so we can't expect too much.
The recording sounds like "yidier" when I was expecting "yidianer." Does the er replace the final n?
The "yidier" you hear is just accent. They are imitating the Beijing dialect. You're pretty much on the money with the er sound replacing the n sound.
Erhua can be used to show that a word is a noun, that something is small and to soften the tone of what is said, ect.
My most common usage of it is in 这儿 (zhèr)，那儿 (nàr) and 哪儿 (nǎr)。(They all mean the same as 这里，那里 and 哪里) Then you have 玩 (wán) -> 玩儿 (wánr: pronounced like war). 小孩子(xiǎoháizi) -> 小孩儿 (xiǎoháir: prounced like xiaohar).
You can here some examples here: https://www.youtube.com/watch?v=VbziFBH8m7w
I take a Chinese Course outside of Duo and the way my 老师 explained it to us is that yi dianer usually isn't said. Usually just yi diar
First character seems to mean "slow", second and third "a bit", so what does 儿 do here? Is "slow" a verb or an adjective here?
The 儿 is already talked about in the above comments. It's called "Erhua" and it makes you sound cuter/ the sentence sound softer.
Here's the link of a video about this phonetic phenomenon which V.Lagopus has given in his comment above:
It can be both, just like in English (eg. "Slow down a bit!" or "the slow cat"). I'm not sure about 儿, though.