Most designers aren’t designing DACs from first principles: for example the DAC chip has already made a lot of these decisions. Even so, many amateur DAC designers make mistakes when doing the analog filters… Most people that have had college courses in digital signal processing understand this stuff, but still some argue about the more subtle details. The deeper details like how reality differs from the ideal repeating waveform usually used in analysis or how to choose the best kernel filter for antialiasing or reconstruction is confusing to many, but those differences are still small compared to what happens when just skipping the filter or skipping upsampling.
Yes, but there are a lot of interpolators: some much worse than others. Here’s a thread about this:
https://forum.psaudio.com/t/upsampling-oversampling-smooth-or-stairs-curve-between-values-determiniation/5791?u=tedsmith
Blurring is a low pass filter function. That’s exactly what’s going on in audio too.