Explain consciousness (viññāna) to non-Buddhists
It sounds like you're trying to explain consciousness within a framework that supports rebirth to people who understand consciousness in a (materialistic) framework that won't allow for rebirth. Seeing consciousness as tied to an organism and the development of consciousness in an organism leaves no room for the "migration" of consciousness required for rebirth to work.

Am I correct in understanding this difficulty?

If so, you could try the following...


Get people to abandon materialism.  For many, this is the same as asking them to abandon science.
Explain rebirth in a materialistic framework.  If consciousness is a property of structure, rather than organism then a rebirth mechanism exists by simply having the same structure arise in the future.  Karmic imprints could be explained by neuro-plasticity (which is said to be affected by meditation).  Still, they may ask what happens if this structure is replicated now (and why wouldn't it be, given how much life we have?).  


An alternative is to look at the big picture.  If you are trying to introduce people to Buddhism, why mention rebirth?  The Buddhist path is identical, whether or not one believes in rebirth.  Further, many are attracted to Buddhism (or at least it's secularized variant popular in the west) precisely because of its less supernatural nature.  Introducing rebirth therefore might cause them to abandon Buddhism.  For many, if it's a choice between Buddhism and Science, then Science will win, no contest.