Did people during the concept and design phase of these anticipate them causing disasters?

in terms of the RBMK? Yes, it was noted in the design specifications that it gets particularly unstable at low power levels due to xenon poisoning. The operators were also aware that they were operating outside of design spec, and not following the recommended operating procedure. They had also remove WAY more control rods than ever should've been removed.

In every possible definition of it, they fucked up in literally every possible way.

Did the people who operate them adhere to best safety practices, maintenance and regulations?

in what way? In most cases, the vast majority of them in fact, yes. This is why only a handful of reactor plants have had issues like this, and the vast majority are operating perfectly fine until this day. Safety practices, maintenance, and regulations are a problem in every industry, so unless you want to argue we should stop doing everything because "bad things happen sometimes" this isn't really an argument unfortunately.

If you need a better example, just look at nuclear powered submarines, there has never been an accident. Including all of the various sinkings that have happened throughout the years.

Did the regulatory authorities ensure that there would be no disaster possible through enforcing said regulations, in particular regarding location specific concerns such as Tsunamis in Fukushima?

So in this case it's more complicated. Japanese culture is a little different to western culture, so this was ultimately a failure of culture, and you can still see the effects of this today with how TEPCO handles itself. At the time nuclear regulation was extremely lax, due to how "formal" it was, so that was the ultimate cause of this problem, but again, this happens in literally every industry.

As long as you have the same human characters in the same economic structures in the same administrative structures, there is no reason to be confident, that these disasters will not happen again.

as long as you have people, and capital, then people should not do anything ever in the potential scenario that somebody gets hurt, or worse, injured, or even maybe killed in an event that shouldn't have otherwise happened* FTFY

And to be clear, the human impact of nuclear energy, INCLUDING all of these disasters is STILL lower than coal, gas, or oil, and on par, if not better than solar, and especially wind.