Poland has every right under the sun to be assisting Ukraine in accordance with UN Charter Art. 51. If they choose to take on Belarus directly if any of the Polish personnel happen to be injured, harmed or get a stubbed toe, ingrown toe nail, then they may do so. There is nothing to stop them assisting a nation that is being attacked in contravention of the UN Charter, which Russia has done. Belarus had no right to assist Russia in the invasion of Ukraine in February, that was a low act. Iran is also in the same boat, they are assisting an aggressor state, and that breaches their basic obligations under the Charter.
Now, if Poland is fighting on Ukraine soil, they are not involving NATO directly. If Belarus conducts armed aggression on Polish soil, then Art. 5 of NATO applies, same for all of the Baltic states.
The tentative response of the West emboldened Russia from day one, we own that bit. There is no escalation under the UN Charter in aiding and assisting Ukraine in Ukraine. That is what Art. 51 expects to be undertaken by nations which are signatories to the UN Charter, where there is clear evidence of one nation failing to abide with the Charter, [Russia], and in the absence of substantive action by the UNSC.
It remains an affront to common decency that the UNSC members do not invoke Art.27(3), as it already removes the right of Russia to vote on any SC matter other than procedural matters, to which tehy also have no veto capability.
It is our governments that fail to have the fortitude to comply with the UN Charter, and in the meantime Ukrainians are dying every minute that our teams fiddle. The Russian conscripts, that are not being paid by Putin, not being fed, provided proper clothing, anything meaningful in body armor, but may get heating from thermal cameras.... No one is being helped by this disaster being protracted by our worthless representatives in the UN. It is not just my position that the UN fails to apply Art. 27(3) correctly, it is the UNSC History library that contends the same.
Whilst I agree with much of what you say re UN Charter I'm not sure that is particularly relevant - many of those clauses are fundamentally irrelevant ever since the day they were written for a wide variety of reasons. I wish it were not so, and I hope that one day they will become more relevant.
However I think your sentences which I highlighted in red are not correct. I don't think a NATO state can choose to become a co-belligerent in a non-NATO conflict, and then invoke NATO article 5 if it is subsequently on the receiving end of a response whether on its own soil, or against its own armed forces. If a NATO stare chooses to engage in non-NATO adventures then the consequences of that become non-NATO in nature.
I'm not wishing to have a dig at you, simply pointing out that I think you've made an error on this particular point.