As a life-long user, glasses with an error of 0.5 are unacceptable for me. That's the amount of change over years when it is time to get new ones.
Not all deviations are measurement errors. There are potential variable conditions of the eye (e.g., spasm of accommodation and sugar level). An online solution that can consider them properly is definitively worth a patent.
Throughout 30 years of wearing glasses, I’ve questioned many times whether the glasses are right for me. I may resist for a month or two but I always end up “adapting” to the new pair. When is this acceptable and how do I know when to speak up?
For A: I don’t really know, but use the ones that gives you B. Depending on the country, high end / non-chain stores can have better customer service and listen to your wishes, and answer your questions. There’s also smoke and mirrors and upselling so see if you trust the person.
B: for simple distance vision (if you are under 40) do a quick pin hole test and look at tree leaves or something. If your vision quality increases a lot you should not be satisfied. Same if you notice eye strain when looking at things in the distance.
Now for multifocal glasses it really depends on your use case (reading, computer work, etc), and it’s very difficult to get a really good correction. There’s no silver bullet. Find an optometrist or optician that you trust, so back to A ;)