Is there a faster way to level up alchemy, or should I just keep creating health potions and poison? Is it faster / more powerful if I use more than 2 ingredients?
The skill gain's dependant on the value of the potion which is generally dictated by the number of effects. Also, some effects are [i]far more valuable than others. E.g. Paralyse > Invisibility >>>>>>>> Restore Health >>> Damage Health. So the optimal way to level is to create potions that have one or two high value effects but with other effects attached - no matter whether they're positive or negative.
E.g. Briar heart + Pearl + Fungal pod = Paralysis, Resist Shock, Restore Magicka, Fortify Block. Even though it's practically useless to you, it's massively valuable and will give you a big skill bump.