Ye Jianying becomes paramount leader-- OTL he was one of the greatest ringleaders behind pushing the Gang of Four and then Hua Guofeng out, and he also persuaded Deng Xiaoping to make his second comeback. If Deng is unavailable he might feel the need to take the job for himself. He also has sufficient street cred within the party as one of the ten marshals.
Well, yes and no. Marshal Ye definitely was the one who had most 'street cred within the party' as you put it, but by 1978, he was not in good health and he knew it, to the extent that he got involved in Deng's machinations they came because he felt first, very strongly that the Gang of Four was outright dangerous for China's future (which, in fairness,
they were), and then that Hua Goufeng was just a weak leader lacking a concrete vision for what to do with his position, and lacking any base in the party to be able to make any meaningful use of it (which in fairness was also true). But he consistently declined to take a more active role. If Marshal Ye takes the job, it's going to be on a purely symbolic basis as a stop-gap.
My money would be on Chen Yun becoming Paramount Leader. His street cred in the party goes back just as far as Deng's, he was there as a loyal member long before the Long March, and he stood loyally by throughout. As an administrator he had established himself as highly competent and pragmatic, and ideologically, much like Deng and Zhou before him, he was very good at saying what he needed to say at times when it was necessary, to shut up most of the time, and indeed, at not saying things that at the time would have been advantageous, but which in the long run would be more of a liability than an asset to have on one's permanent record.
Hu Yaobang's career was largely dependent on Deng's, so he probably doesn't get to be Premier. That job might go to Chen Yun, OTL involved in managing the economy since the 50s and critic of the Great Leap Forward. He'd find Zhao Ziyang to be too much of a free marketeer to be handed the reigns over the economy. Li Xiannian is another possibility.
China's economic growth ITTL probably ends up slower than OTL in the 80s, and SEZs don't exist (Shenzhen is just this small town Hongkongers get their cheap goods from). I also feel that the US is less likely to turn a blind eye if anything like Tiananmen happens here, which either means a) China is knocked back into autarky or b) the government falls apart and a reformist local politician takes over, maybe Li Ruihuan.
You're definitely right that Hu Yaobang and Zhao Ziyang's careers are going nowhere. They were both far too dependent on Deng to get anywhere meaningful, and Chen Yun, who regardless of what his precise position will be will still be one of the major players, absolutely detested both of them, and without Deng being there to shield them, yeah, their careers are dead in the water. They won't get purged or sent to any labour camps or anything, there just wasn't an appetite for that any more. They'll just no longer be relevant politically.
As for Tiananmen, well, it's going to be interesting. Bush and Deng knew one another since the 70s, and while it would definitely be a stretch to say that they were
friends, they did have an amicable relationship, with both men feeling that the other was someone you could trust and do business with. Indeed, before the blood had even been cleaned up from Tiananmen, Bush called Deng and basically said, 'I'm sorry that we're going to have to put a few sanctions on you, old boy, but people here in the States have gotten rather upset. Still, don't worry too much about it. In a few years, people will have forgotten all about Tiananmen Square, and then we can go back to business as usual.'
So it's not just the fact that Deng was the fellow in charge in China when it happened, it's also the fact that the President of the United States at the time happened to by nature be inclined to look the other way vis-a-vis China.
Of course, without Hu Yaobang being there in the first place, you're not going to get Tiananmen Square, so, well, there's that.