Aside from the points raised in other answers, I think the comments you posted contain some questionable reasoning. I'll quote the comments here and explain why. I'll wrap it up at the end to make it a proper meta answer rather than a continuation of the discussion. ;)
I think the summary of the situation you give is approximately an inversion of reality (as I understand it). For instance, "Russia stole Crimea from Ukraine": every one of the more than a dozen polls conducted by international- or Western-based institutions in the years before, during and after the 2014 Crimean status referendum found that a substantial majority of Crimeans wanted to join Russia, or believed the referendum reflected the will of most Crimeans, or felt that life had improved since reunification with Russsia.
Cont.: Russian military were present in Crimea by right of a 1997 treaty and were carrying out additional peace-keeping duties by invitation of then-President of Ukraine Viktor Yanukovich. Was the referendum technically legal? There's some room for debate on that: it depends on whether or not Crimea was justified in first declaring independence, which depends on whether or not it could expect its rights from the new, unelected, putschist NATO client government in Kiev. Was it legitimate in the sense that it expressed the will of most Crimeans? I think there's little doubt that it was. –
That Russia "decided by themselves without outside reason or provocation to invade the country and make it a hot war" is bizarre. The OSCE observed a dramatic escalation of shelling by Kiev forces into the Donbass in the week or so before 24-02-2022, understood by Russia as a sign that the NATO army in training in Ukraine since 2014 was now prepared for an offensive aimed at retaking Donbass and Crimea. If Crimea is part of Russia's sovereign territory, as Russians and Crimeans believe, then this means Russia and the Donbass republics had a right to pre-emptive, collective self-defence. –
Former President of Ukraine Pietro Poroshenko has since publicly acknowledged that the Minsk peace process was, from the Kiev-NATO POV, a sham aimed at buying time to build up Ukraine's military power for just this purpose. Individual and collective self-defence is promised by Article 51 of the UN Charter; pre-emptive self-defence is part of customary international law as the Caroline Doctrine; pre-emptive self-defence by a collective security organisation (here Russia and the Donbass republics) is the doctrine crafted by NATO to justify the Kosovo intervention. –
First of all, I would say this amount of text is more than just pointing out some errors in the answer. This is either a new answer taking a different direction (i.e. competing with the answer you're disputing) or it's a start of a new discussion. I don't think it directly address the question, so I wouldn't post it as an answer.
As for criticising the answer, I would suggest picking one assertion the answer makes and then disputing that.
As for the contents of your comments, I don't think the reasoning holds up. First you seem to be overstating the relevance of the 1997 agreement. That agreement contained among other things (as summarized by Wikipedia):
Russia was bound to "respect the sovereignty of Ukraine, honor its legislation and preclude interference in the internal affairs of Ukraine" and, furthermore, Russian military personnel had to show their "military identification cards" when crossing the Ukrainian-Russian border; Russian forces could operate "beyond their deployment sites" only after "coordination with the competent agencies of Ukraine.".
Clearly annexing Crimea violates the above. Russia cannot just hold a vote in a foreign country and then say the territory is theirs because the people voted for that. It also violated the Ukrainian constitution which said that a referendum would have to be held across the whole of Ukraine. If only Russia was bound by some treaty that it would respect Ukrainian law...
Your second point is that Russia, Donbass and Crimea are afforded some right of self-defense against Ukraine. That reasoning doesn't hold because Russia and those self-declared republics only control those areas because they took them illegally. That's like stealing a car and then calling the cops because the rightful owner tries to take it back.
You are also referring to Article 51 of UN Charter, presumably to say that Russia was allowed to aid those break-away republics to defend them against Ukrainian retaking efforts. That does not work because Article 51 applies only to attacks against a Member of the United Nations. For this reasoning to apply, the break-away republics would have to become UN members first after which they could ask other countries to come to their self defense.
Instead, those break-away republics are part of Ukraine and Russia is the aggressor. As such, Ukraine has the right to self-defense per Article 51 of the UN Charter and that's the legal framework through which it has been able to ask other countries to aid in the self defense.
Let me quote André de Hoogh who is a professor of International Law in his article entitled The Elephant in the Room: Invoking and Exercising the Right of Collective Self-Defence in Support of Ukraine against Russian Aggression (click the link if you want to follow up on the case law cited in the article):
As to the question of a legal right to act militarily in support of Ukraine, States are generally prohibited from using force against each other under Article 2(4) of the Charter of the United Nations and customary international law (Nicaragua case, para. 188). As such, the position taken up by NATO and Western States is consonant with that prohibition. However, it must be noted that Ukraine has the right of self-defence against the invasion by Russia. That invasion satisfies the conditio sine qua non set forth in article 51 of the Charter, and required also under customary international law (Nicaragua case, para. 195), namely that an armed attack occurs against a member of the United Nations or a State. Furthermore, the invasion of Russia satisfies the gravity requirement, in terms of scale and effect, which distinguishes an armed attack from a (‘mere’) use of force (Nicaragua case, paras. 191 and 195). Indeed, it also satisfies both the qualitative (character; animus aggressionis) and quantitative (gravity and scale) requirements for being considered an act – nay, a war – of aggression (Article 8 bis Rome Statute of the International Criminal Court; note that neither Russia nor Ukraine is a party, but that Ukraine has made a relevant declaration under Article 12(3), accepting ICC jurisdiction for acts committed since 20 February 2014, which will not however extend to the crime of aggression).
Back to your comments, I think this answer shows why comments are unsuited for this extended discussion of complex legal matters. If you can point out one or two mistakes in a post and address them in one comment then it can be resolved. If it requires multiple comments or even a full answer to do so then it's going beyond what comments are meant for on this site. Another problem is that comments cannot be downvoted, you might be able to make a compelling argument that doesn't hold and users can only dispute it by adding more comments.
So my suggestion is as follows:
If it only pertains to one answer that you're disputing: keep the comments short and specific. Ideally one comment to make your point.
If it pertains to an answer but your dispute can also answer the question: post it as a competing answer. The benefit is that users can vote the answer up and down, the character limit is way bigger and others can edit to improve your answer.
