For example, in 売り切れ, do the り and れ have a particular name? I found "okurigana" on wikipedia, but does that apply to grammatical suffixes only or does apply in these cases where it's mixed in the middle with kanji too?
2What Peri461 said in his answer. Further, In your example of "売り切れ", 売り is an inflection suffix of 売る, just like 切れ is an inflection of 切れる. So these are not kana "in between kanji"; they are characters trailing each kanji, i.e. they're okurigana. り is "in between" only incidentally, because in Japanese you can glue two verbs together; but it's still a suffix (just like, in English, 'sensationalism' has two suffixes; '-al' from 'sensation-al' is still a suffix, even if followed by another suffix '-ism'). In 売り切れ, り is a trailing kana (okurigana) of 売 and れ is a trailing kana of 切.– melissa_boikoJun 13, 2017 at 13:25
Yes, you're essentially right. 送【おく】り仮名【がな】 is the name for kana used in conjugations of verbs and adjectives. There might be another name I'm not aware of, but I haven't heard of one just for kana in between kanji.
See here for more about 送り仮名. (Sorry, I entirely forgot you said you had already seen Wikipedia already.) Anyhow, 送り仮名 does not concern kana that just happen to be in between kanji. Those would either be particles or other words entirely.