from Nemea → Νεμέαθεν, Νεμέηθε, Νεμέηθεν

[spiros]

Adv. Νεμέαθεν, poet. νεκῠό-ηθε, from Nemea
Νεμέα - Ancient Greek (LSJ)

Should the poetic form be Νεμέηθε/Νεμέηθεν;

[billberg23]

Definitely, the only form should be Νεμέηθε(ν), since it's attested only in Callimachus fr. 103 and in Stephanus Grammaticus (6th cent), who states Νεμεῆται. οἱ πολῖται τῆς Λοκρίδος πόλεως Νέμειοι. καὶ Νεμέηθεν ἐπίρρημα.   Νεμέαθεν is only hypothetical, as an imagined Attic/koinē form. 
BTW νεκυό-ηθε (Wiki) seems to be a misprint for Νεμέηθε, since it's in neither the 1940 edition nor the online LSJ, and anyhow makes no sense. 

[spiros]

Perseus LSJ has the same. Which online version where you referring to? There are multiple. The wiki was based on Perseus.

[billberg23]

Ah, so Perseus is the culprit here.  Callimachus, fragment 103 (= Plutarch, Quest. Conv. 677b1ff.), reads as follows:
    καί μιν Ἀλητιάδαι, πουλὺ γεγειότερον
  τοῦδε παρ' Αἰγαίωνι θεῷ τελέοντες ἀγῶνα,
   θήσουσιν νίκης σύμβολον Ἰσθμιάδος,
  ζήλῳ τῶν Νεμέηθε· πίτυν δ' ἀποτιμήσουσιν,
   ἣ πρὶν ἀγωνιστὰς ἔστεφε τοὺς Ἐφύρῃ.
No one, as far as I know, suggests a variant reading for Νεμέηθε.  The word νεκυό-ηθε, besides making no sense, won't fit metrically.  Here is the original 1940 LSJ print version:
Adv. Νεμέαθεν, poet. —ηθε, from Nemea, Call. Fr. 103.. Same in Diogenes' online version.
Even Homer nods, now and then.  So does Perseus.