The Chinese constitution for example was change by Deng Xiaoping, guy right before Xi and then changed back by Xi. Not much of a 'constitution'.

? I can't tell if you're saying that Deng Xiaoping was the guy in charge right before Xi (which is just wrong since he died like 30 years ago), or if it was changed by him and the guy before Xi (in which case you could have just named him...), but either way, I'm not sure how this supports your point. Really, all it does is make China's laws sound arbitrary. In any case, the fact Xi did have to remove the term limits added to the constitution in the 80s is the point.

The point is that you're taking an arbitrary condition which applies to Russia and China and doesn't apply to Ukraine. But similar arguments of Zelensky staying in power usin martial law can be made. Zelensky is extremely unpopular in Ukraine currently and would almost certainly lose elections if they were held.

I feel like this is a pointless argument, but using the existing constitution and laws to declare martial law while in a war on your own soil is very different from arbitrarily changing the constitution of your country to allow you to rule forever. If the war ends and he remains in power, then it will be a different story.