Hadn't heard of it. Looked it up but couldn't tell if it is grafted... but seems it's supposed to be a single stem. So yours is going to be a potential problem in that 2 are going to be weaker. I really don;t know if taking out one side-shoot and bending the other up with a stake will work but that's my only suggestion other than completely replacing it. Did you keep the label to know what nursery it came from originally? I'd be inclined to go back to the source and request a replacement. BUT even though it's supposed to be "small" that seems to me a planting that's also hazardous - very close to the fence with a lot of reflected heat?

Some of the WA gum trees are regarded as weedy in Victoria because they hybridize with the local species and 'polute' the gene pool. Corymbia/Eucalytpus ficifolia is OK as far as I know. The only local Eucalypt that grows to a moderate size (12m), that I know about, is pauciflora or Snow Gum. Yes they do grow around Melbourne though they are not common. If selecting local species and you have the space for then stick to the Box or Stringybark Eucalypts, as these retain all their bark and generate very little mess. Although you could probably get away with a River Red Gum which also grow to around 12m. By the time they reach their full size and start sheding copius amounts of bark and limbs you and I will have departed this earth.

The best treatment is to ignore them, they are a natural occurence in Eucs and may be due to organisms such as wasps, flies, beetles, psyllids, coccids, moths, nematodes and mites as well as bacteria and fungi. They rarely cause significant damage to a healthy tree, although stressed, isolated trees may be different. The best control is a healthy yard with plenty of predatory birds and insects. Systemic insecticides are impractical on large trees, usually inneffective, almost always uneccessary and more like to harm off target species than the gall forming organisms. You'll probably find as your Euc grows up you hardly notice the odd gall.

Yeah, you'd be ok for a few years, just be careful with any fertiliser you might be thinking of adding - in fact If it was me I wouldn't add any to a pot grown native. In a general sense, it'll be ok for a few years until it fills the pot with roots and as they hate root disturbance, that'd be about as far you could go with it. However, you could try to bonsai it which would require a bit of study on Natives for bonsai culture. I'd be thinking along the lines of while repotting it each year, doing a very brief pruning of the roots, perhaps just on a third or less of the root ball each year along with ongoing top pruning. That way you might be successful.