The references to Rashi, Raavad, and R' Avraham ben haRambam* are explicated in Otzar Yisrael (and from there in the Daat Encyclopedia):
Rashi - to Prov. 5:19 cites an explanation of the word תשגה in the name of R' Moshe Hadarshan, who in turn bases it on an expression used by Eldad. In the area of halachah, Rashi (Pardes, Hilchos Treifos) accepts Eldad's explanation of why a delay (shehiyah) during slaughtering invalidates the animal.
Raavad - the reference is to Raavad II (R' Avraham Av Beis Din), author of Eshkol, who cites a couple of halachos of Eldad's (he calls them הלכות ישנות). He disagrees with some of them, but accepts one as a possibility.
R' Avraham ben haRambam - mentions Eldad's stories in a responsum of his (published in Kovetz Al Yad 4 (5648), pp. 62-63) as proof that the Ten Tribes are still extant and identifiable.
In the area of practical halachah, for the most part his statements (almost all of which refer to the halachos of kosher slaughter) are rejected as normative, though sometimes the posekim accept them as a stringency. For example, Eldad states that shechitah without a blessing is invalid; the accepted halachah is that it is valid, but Taz (Yoreh De'ah 19:1) cites the Bach who says that if the person did so knowingly (and didn't just forget to recite the berachah), then he himself is penalized by not being allowed to eat the animal's meat. As another example, Eldad rules that a person who has not immersed himself may not perform shechitah, and R' Yonasan Steiff cites his teacher, who insisted that the shochetim under his authority indeed immerse themselves daily (tevilas Ezra).
All told, then, we basically follow the lead of R' Tzemach Gaon, with whom Eldad was contemporary, who wrote that there's no reason to reject what he says out-of-hand despite the seeming strangeness of what he says, and certainly no reason to think of him as a fraud; each of his statements has to be examined on its merits.
[This R' Tzemach was Gaon in Sura sometime in the late 9th century - there are various views among historians (mostly due to variants in R' Sherira Gaon's letter, which lists the Geonim and their tenures) as to the exact dates. In any case, yes, Eldad made his appearance sometime in that period.]
* Yes, that's who the Wikipedia article means. It seems to be fairly common among scholars to refer to him by his grandfather's name, e.g., "Abraham Maimuni" - perhaps because Maimon is an uncommon name in Jewish history while Moshe is fairly common.
