Goodwin, in Greek Moods and Tenses, indicates a fluidity with regard to
person and number in use of the imperative. 251: The imperative is often
emphasised by AGE or AGETE, FERE, IQI, DEURO or DEUTE...; or by EI
D'AGE... AGE, FERE, and IQI may be singular when the imperative is
plural, and in the second person when the imperative is in the third.
E.g. ... ALL' AGE MIMNETE PANTES, EUKNHMIDES AXAIOI. Il. ii.331... AGE DH
AKOUSATE. XEN. Ap. 14. ... DEUTE, LEIPETE STEGAS. EUR. Med. 894.
254: The imperative sometimes expresses a mere assumption, where
something is supposed to be true for argument's sake. E.g. ... PROSEIPATW
TINA FILIKWS hO TE ARXWN KAI hO IDIWTHS. suppose that both the ruler and
the private man address one in a friendly way. XEN. Hier. viii.3.
Regarding the absence of EINAI and a possible semiticim, Il. xvii.
179-180, where the verb to be IS EXPRESSED, may be of interest to this
discussion:
ALL' AGE DEURO, PEPON, PAR EM' hISTASO KAI IDE ERGON,
HE PANHMERIOS KAKOS ESSOMAI...
I guess the point of this is that there is ample evidence, from Homeric
times through the Hellenistic period of such variety and flexibility in
the use of Greek imperatives that it belies our expectation of a "normal"
singular or plural "command" having a corresponding subject in agreement
as to number and an anticipated object in the accusative case.
I guess I'll close by repeating that I think that in this instance IDE is
best understood as an imperative that has become frozen in the idiom as
an interjectional usage.