Chances of Septum Deviation Due to Memory?

 The chances are small but some noses will re-deviate but only slightly if at all. It sounds like your surgeon did a good job, so relax and don't worry.
 

The septum can shift through memory in the first few months after nasal surgery. The chance of it shifting is probably in the 5% range. This really depends upon what the septum looked like preoperatively, as well as the techniques used to straighten it.

It is well known that cartilage does indeed have a tendency to warp. We routinely do a host of maneuvers to minimize this risk: strut grafts, battens, fixation the septum to the nasal spine, etc.
In most cases, any warping that occurs is thus minimized. Your nose is not going to end up perfectly: this never occurs. But, it is likely to heal well and you should be satisfied.
Occasionally, however, despite ones best efforts, the cartilage will warp some. This is very frustrating to the patient and the surgeon. It can occur in days, months, or even years after surgery. Fortunately, even in these cases, experienced surgeons can correct this with a revision procedure, more often than not.

It depends on the technique used in the septoplasty. Most commonly a septal resection is used which removes the deviated portion of the cartilage. This way the deviated septum should not reccur. Scorring of the cartilage has less effect and might reccur but it is also used in milder cases.

I also had a 3 hour open septoplasty (no rhinoplasty) 8 weeks ago today. The resulthas been great. The chance of your cartilage warping and re-obstructing would depend on what was done. In my case most of the cartilage was removed leaving only an "L strut" to support the dorsum and columella. A columellar strut graft was also placed for further support. I also had spreader grafts and the small remaining piece of cartilage was replaced between the leaves of septal mucosa to minimize baffling on inspiration.
Sometimes, the septum is retained and repositioned and I would think this might have a higher chance of warping but you should ask your surgeon for the details of your surgery. Good luck and happy breathing!