I read it as you need a free hand to cast (or manifest or whatever the proper verb is for this) but only when casting it. Once the invocation is invoked you hand is free to use the weapon in question.
That is my interpretation, too.
In general WotC has said (via FAQ and other things) that "as if" in general means just like.
True. and the word "as if" is only used for "wielding a reach weapon". Not "as if wielding a glaive" or "as if you are wielding a two-handed reach weapon" or something.
So I think the pseudo-reach weapon created by Eldritch Glaive does not require two-hands.
Combining those two interpretations, I say one can perform necessary somatic component when he use this invocation, then can use that same hand to "wield" a (non two-hand) pseud reach weapon.
There may be 3 other possible interpretations.
If you can use the same hand for performing somatic component and wielding the pseudo-weapon, but the pseud-weapon is a two-handed weapon, then you need 2 free hands to use Eldritch Glaive.
If you need a free hand all the way during a full action for this invocation but the pseudo-reach weapon is a one-handed weapon, then you need 2 hands to use this invocation. One to perform a somatic component, and another one to wield a one-handed weapon.
If you need a free hand all the way during a full-round action for this invocation and the pseudo-reach weapon is a two-handed weapon, then, you need 3 hands to use this invocation. One to perform it's somatic component and 2 more to wield a two-handed weapon. Thus usual humanoid warlock cannot use this at all.