Is NATO using Ukraine to launch weapons into Russia justified?

There is a bit to unpack here because the title is phrased in a confusing way, and I want to clarify every single part of it.

First the title implies Ukraine this is a proxy war between NATO and Russia, where NATO is using Ukraine to attack Russia, and that’s just not the case. NATO countries are helping Ukraine with weapons and ammunition the way China and North Korea are helping Russia; that does not make it a proxy war. If simply providing military help was the criteria to make a proxy war, this would be a proxy war between NATO and North Korea.

Secondly, it is the US that had concerns about Ukraine striking inside Russia, all other NATO countries had approved this with no issue. It’s a remnant of their political gridlock, which is why it was approved so late.

Ans finally, this is the answer you wanted: yes. Any form of attacks on military targets that stalls or stops the invasion is a valid mode of self-defense for Ukraine. Like you said, this is not civilian centers.

Is Russia using North Korean (who are UN sanctioned) and Iranian weapons and missiles into Ukraine Justified?

As usual for Russian geopolitical carry on, play stupid games win stupid prizes

Once Russia invaded Ukraine unprovoked, Ukraine had and will continue to have the right to strike Russian military targets anywhere in the world until either Russia retreats and de-escalates, or a peace deal is reached.