It was slower and harder to do than with digital editing, but it was absolutely common-place and normal.

As someone who grew up in that era, I can say all the family photos have not been edited, nor would the majority of people who owned a camera back then spend the time or money to do so. Yes, commercial photo editing services have always existed, and of course you'd expect things like magazine covers and other prominent, publicly published photography to be heavily edited, but they didn't automatically do it to everyone's photos for free; and that's the huge difference between then and now.

Honestly I was thinking more of the "portrait studios", rather than the home-shooter. Though I guess in teh early days of cameras there were a lot of people who did their own development at home, in ad-hoc darkrooms. Maybe they didn't edit so much, but it was within their means, and I'm sure it was more commonly done than we'd suspect.

Whoever developed your photos would've adjusted the brightness and white balance based on what looked right to them if nothing else. Send the same negatives into two shops and you'd get different-looking pictures back.

It was an automatic process and colours were consistent. I find that reading theories about what might have happened 20 years ago is becoming pretty annoying. I shot some kind of Fujifilm mostly and if it was under/overexposed that was like your problem.