the overmind covering an entire planet sounds a bit like "Solaris", a very good SF book
i'd like to devise some cross-links. shut me up if i got it all wrong...
ok,so the overovermind's portion (==overmind) sent out to gather food was inactivated for quite a long time - it originates back to the complete eradication of all life on mars centuries/millenia gone. since then, is has stayed there until its next order from the overovermind comes floating in (or, it realises the neighbour planet has become real yummy in the meantime). As it is, the fully equipped war machinery which was stationed there to harvest mars has left a while ago to fight its full-scaled war. So, the Overmind lacking them, starts abducting humans for fresh material and clones also... well we all know where this leads.
The essential part is, that the Overmind was stripped dry of resources. That would certainly explain the first trickling waves of ufos which increase steadily. It would also explain the fear-by-terror tactic (to avoid serious resistance leading to general failure from the beginning) and the divide-and-conquer tactic (getting the human governments to collaborate --> no resistance). As signs of weakness, those tactics are abandoned later on when the alien force becomes awesome once more.
well, my brainstorm ideas for the diverse alien origins:
first, aliens who are sent initially to earth which have been enslaved by the overmind a long time ago - the real background of those aliens will be considerably hard to find out because the race itself has been sent to and fro and been twisted and reworked upon all the time. As a result, maybe the true nature of these aliens can only be determined if the Overmind is put to the question or a real highranking alien is freed from the overmind's psi control and remembers fragments of their race having been free and peaceful once...
second, the "aliens" which are constructed by human material to fight are clearly identified by autopsy - it's rearranged human flesh (shocky!)
third, aliens which aren't really controlled by the overmind, only steered and in reality driven by an own hatred for humankind can be interrogated where they eventually reveal their personal hatred (like the race in TFTD - uh, only know in german: Kiemenmensch - help me out there)
fourth, aliens which were originally "pets" or partners for enslaved races and simply cannot be used without their masters and vice versa (because they form a formidable fighting team). Since those aliens defer naturally to their real masters, the Overmind only needs to steer them. Their nature will be revealed by analysing their live behaviour - and the homeplanet by interrogating their masters (well could be the same or an invasioned planet in the past)
that's about it folks, tell me what that sounds like.