I went with the second option (half caster mix of invocations & slots) but would take the third one (3.5 style not a caster no spell slots invocations only). The deciding factor was that the 3.5 style one depended on factors no longer present that I have zero expectation of seeing like the 2/3 BaB it had. Other differences are things like the fragility of casters stacked against the much higher HD & armor capabilities of the base warlock that get into wide ranging system differences.