As I said, if you play for 2 hrs a night and you use only one "rest cycle" per playing time it will take you 4/5 days to complete the order.
If you devote 2 hrs RL to brewing yes, you will be able to fulfill the order in just one night of play but you will be "hampered" in your questing ability.As per using the unused spells, it really depends on your spell selection. I rarely end up with a spare shield spell (I don't use it since I have a few spare potions on me), invisibility (same reason, I rarely use a spell slot to study invisibility if I have potions) or blur (which can only be brewed if you have the non-purchasable recipe).
Now, personally, I have a lot of time on my hands, from a combination of summer break and my college deciding not to hire student workers in my department over the summer. Now, I'm sure that if I sat down and made a spellcaster, I could probably squeeze out a small profit should I decide to sell potions, but only because I have absolutely nothing better to do with my time. However, someone who doesn't have loads of time to devote to sitting in a chair playing Arabel, such as yourself, probably should not be devoting so much time and effort into making potion brewing profitable on such a large sale.
To me, the message I'm getting is that you'd like to see potion brewing become more accessible to people who don't have obnoxious amounts of time on their hands. Given what's been said, though, your most likely alternatives are either to start crafting wands, or stop trying to make potion brewing profitable for yourself.