Temp going up when sitting in traffic is normal - if you smell that sickly coolant smell it certainly sounds like the heater core. At this stage I'd decide if I want to save money or spend some plus time. If the former just add some stop leak to the coolant which will slow/stop the leak for a few months, maybe a year. If the latter, you can replace the core without pulling the dash and total outlay doing it yourself will be under $200.
Other alternative is to bypass the heater...just block the pipes. This is OK if you are north of Brisbane.
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