Why have you been issued an auto if you've had a titration study? Sounds like rather a waste of time.
That can mean a few things, the most common of which is the memory card being in the wrong way around. The memory card should be inserted with the chip end first and the coloured side facing away from the machine. If that doesn't make sense, just try every different orientation until the beeping stops. There are only 4 different ways to do it, so it shouldn't take too long. Failing that, it probably needs to be repaired. The manual should give you some idea.
According to the most up to date research I am aware of, the odds are good that if you snore you have sleep apnoea too. So it's worth getting it checked. It wouldn't surprise me if Bose never even thought of it.
My current favourite intranasal mask is the DreamWear from Respironics which comes in both intranasal prong and under-nose flavours. ResMed has just released the N30i which is their rip off the DreamWear. I have not had a chance to try it yet so I can't give you any first hand impressions.
No, a deviated septum does not cause OSA as far as I am aware. However, being young, fit and slim doesn't discount you from having OSA. It is certainly more common in people who are older and overweight, but it can occur at any age (right down to neonates) and to people of any weight. It looks like you just got lucky.
On this topic I can respond confidently as the expert! What you have posted there in images 2 and 3 is very typical obstructive sleep apnoea as recorded by a PSG. It's not all the data that a full PSG would give, for example there's no EEG, so technically I can't be sure if you are awake or asleep, but that pattern is very typical of OSA. The degree of oxygen desaturation is not that unusual. I imagine your GP couldn't explain it because he doesn't know how to read a PSG, or see a lot of them. Nor should he because it's not part of his job. Image 1 looks to be the same thing but on a really large time scale. Image 4 appears to be mostly awake (although again, without EEG it's hard to be sure), possibly trying to back to sleep post arousal, so I wouldn't worry too much about that.
Next step, CPAP.
EPR lowers the pressure a little as you breathe out and raises it as you breathe in. The setting of 1, 2 or 3 controls how much it does this with 1 being the least and 3 being the most. It is designed purely as a comfort measure, but if used injudiciously can cause problems.
Sounds like it would be worth you while having a sleep study.
Yes, it is very possible that a relaxant medication can increase your tendency to obstruct. It is a well-documented phenomenon. However, 4.5 is still very low, so I wouldn't get too worked up about it.
If you are a little more mercenary you could probably sell it quite easily. There's a roaring trade in second-hand CPAPs.