14/3 Wire + Double Pole Breaker, Two 120v Circuits?
Well it IS a MWBC. It just happens to be one that serves one outlet on the one side, and (presumably you've checked that the other outlets do actually turn off when the breaker does) the remainder of the outlets and the lights from the other half. It still economizes the wire from the panel to the room (one 14/3 rather than 2 14/2's.) 

The older 12Ga wire presumably just means that the older outlet wiring that was not changed out was done with 12/2, and might have been on a 20A circuit at that time. Since it's more than large enough for 15A, that's perfectly safe in the current arrangement. Likely the two runs of wire are in, and on to the next device in the chain. 

Personally, I find the combining of outlets and lights to be a thing I object to (since I don't like getting left in the dark, and fixed lights almost never trip a circuit breaker, while outlets get whatever plugged into them until they do) but electrical codes in both of our countries don't care about that.