The fact that the transition takes a very long time isn’t proof that it isn’t transitioning. What even is this assumption that transitional periods must last less than a decade? Seriously, where the heck does that even come from?
To answer your question, this transitional state is necessary as long as capitalism remains the overwhelmingly dominant mode of production on the planet because in a mainly capitalist world, transfer of technology and resources mostly happen between businesses doing business.
If you try to go to a higher stage of socialism while the world is still almost only capitalist you’ll end up with all the problems that plagued the soviet union, with the capitalist countries able to very easily sanction and isolate you since they can’t get access to your markets even if they don’t anyway and with you having to re-invent every new technology the rest of the capitalist world create just to keep up since there is no way the capitalists would give you the blueprints among other problems.
There are several. The private sector has never dominated the economy, the public sector always kept a firm hold on banking, raw materials, energy production and infrastructure that the private sector is dependent on to make and deliver what they sell, in other word, a massive leverage the state can use to pressure the private sector.
They can literally starve private companies of financing if they want, which they did when they let real estate speculators go bankrupt after the state voluntarily burst the real estate bubble. Something a bourgeois ruled capitalist country would have never done.
Moreover since a few years ago, the proportion of the economy that is privately owned has been decreasing while the state’s control over them has increased.
Here is a video explaining China’s socialist system in which some such evidences are presented.
That’s still very arbitrary.
I’ll let answering this one to someone with more more knowledge on 1922-1925 period. I’ll only say that Lenin never tried to prevent Stalin from taking power. The Lenin testament, assuming you are at least partially referring to that, is most likely forged. We know from Lenin’s numerous letters and other writing that Lenin had an extremely poor opinion of Trotsky and his politics, and as such would have never recommended Trotsky as a potential general secretary of the party. Furthermore, Lenin and Stalin were close friends.
They can be voted out of their position. Literally.
The political system in China, to put it very simply, is a bottom up elected council system. The peoples vote for local administrators like mayors and such, these local administrator vote to elect the rank above them, who themselves vote in the ranks above them and so on all the way up to the congress general secretary (side note: Xi is both the president and the general secretary, but the president is a largely ceremonial role and doesn’t have that much power, Xi’s real political power comes from him being the general secretary, no from him being the president).
And for each rank, the elected officials can be un-elected by the ranks bellow. Even Xi could be un-elected, he won’t because he is very popular among both the peoples and the party members, but he could be. This is one of the rational behind why they removed the terms limit by the way, why have a time limit that automatically end the general secretary’s term when he can be un-elected at any time?
Yes, as I said, in a capitalist world exchanges between countries are done mostly through businesses. So in order to have exchanges of resources and technology and not be cut of and starved like the USSR was, having businesses selling to other countries and businesses coming to sell in yours is a necessary evil.
Although, China has been reducing their exchanges with the US for almost a decade now, and it is only accelerating with Trump’s lunacy. Right now, Chinese money is overall leaving the US, not entering it. China is now a net seller of US treasury bonds instead of a net buyer like it still was until relatively recently. China also banned the export of a lot of dual use metals, especially rare earths, to the US. And since China controls between 30 to 90% of production depending on the specific mineral, the US can’t really get those from anywhere else.