How to reference existing term if it exists or create new one if not exist via Inline Entity Form module?
This solution is a bit "out there" so I'll just describe the logic. 


Three taxonomy fields. One for first name, one for last name (so there's autocomplete), one for full name generated by rules.
On node save, join the first+last as tokens and fetch term by property name. This will give a list containing one or no terms.
Two rules components that receive the list.
No. 1: condition, list is empty. Actions. Create term and send it back to the rule.
No. 2: condition, list is not empty. Actions. Send back the fetched term. 
In the main rule, set data value on node, returned terms. 


It's "out there" but that's because there are two fields for one term.