Couple this with the Manchu confederation breaking apart (say Dorgon dies in a riding accident or somesuch) and you effectively end the threat of a Qing invasion of China.
I agree with you about Wu Sangui and Li Zicheng, but I think that even in the event of the Ming being replaced by the Shun rather than the Qing, China would remain vulnerable to Manchu encroachment on its northern frontier. The Manchus were a rising power in north-east Asia and, like other partially Sinicized steppe nomads before them, would likely have attempted to expand south whatever else happened. The Shun would have to deal with them one way or another, and it's an open question whether they'd have time to consolidate their power in the home provinces before turning their attention to the North.
There's also an interesting side question: how far beyond the home provinces would Imperial rule extend without the Qing's military expansion in the 18th century?
It's also possible that the Shun would, like the early Ming, seek to assert their legitimacy with an ambitious program of maritime expeditions, making a name for themselves, as it were. This being the mid-17th century, and the trans-Pacific routes to the Americas being well-charted by the Spanish, they may even decide to join the Western powers in making territorial claims in the New World, you never know...