Welcome to medicine. I grew up the child of a doctor, and she always, ALWAYS, bought the generic. They're required BY LAW to have the same "active" ingredients; you just might be paying for a slightly different pill shape, packaging, or taste.
But actually, one advantage of brand name drugs is that it could be easier to remember what they do. For example, sometimes I find old prescriptions in my medicine box (yeah, I don't have a good cabinet) and I don't remember what they are for. For example, I might see Zolpidem and and forget what is, but I'd be less likely to forget what Ambien is. But in general, this difference is worth very little to me and I usually get generics on principle.
The law states the generic should be within 80% to 125% bio-equivalent of the branded version. Normally that's good enough, but for some drugs it causes problems.
Yes, but the margin of error of the amount of the active ingredient can be significant (up to 20% in either direction!).
The inactive ingredients can be relevant. Some people have allergies[0], and sometimes the inactive compound can affect the absorption of the drug (which can vary from person to person).
Generics are fine, but it's important to remember that they're not exactly the same as the brand-name drugs, and that can matter.
[0] For example, a lot of drugs contain lactose-based compounds as inactive ingredients, which is one of the reasons that a lot of drugs can cause diarrhea as a side-effect.
It's also why some people who are genetically completely lactose intolerant can actually consume quite alot of lactose, and conversely why people who are genetically lactose tolerant can suffer from gastrointestinal issues pretty easily from dairy products, especially if they haven't consumed them in awhile. The tolerance or intolerance boils down to how the microflora react, not specifically to whether the person produces lactase.