I have strong doubts that "Catholic Ming warlord state based on naval supremacy" leads to a conquest of China.
Oh yeah, Koxinga was Catholic. Yeah I guess his Kingdom taking over China is unrealistic, but I wonder if him doing much better could subject the early Qing to a cumulative death by a thousand cuts of rebellion, that some other group later exploits.
Possibly the Three Feudatories, especially Wu Sangui
As for a Russo-Chinese war, I could see this leading to bad trouble for the Qing if they lose. PoDs could be with a) Qianlong underestimating the Russians and launching a campaign of aggression in response to Russian slights in the 1750s-1760s or b) Catherine the Great possibly launching a campaign of aggression in the 1770s, possibly using Qing interference with her Mongol subjects as the excuse. More detail previously posted on SHWI:
"
...But tensions did exist, war was possible at times, and it is
interesting to speculate how the Russians and Chinese would have
stacked up against one another during the reign of Qianlong
(1735-1799)
---I see two main scenarios for this, an earlier one based on Manchu
aggression, and a later one based on Russian aggression. I welcome
tweaks for making a conflict more realistic, and thoughts on the
sides' relative military capabilities and performance, and their
possible diplomatic and strategic objectives against one another.
Scenario A) Qianlong determines on a campaign to reduce Russian power
and chastise the Russians, in the tensions brought on by China's
demands for the return of the body of Dzhungar chieftain Amursana (the
campaign against Amursana was what in OTL resulted in the Qing
Dynasty's conquest of Xinjiang and Tibet), and Russia's refusal to
return the body.
Tensions were fairly high starting from 1758 for the next few years.
In OTL, the Qing threatened to cut off trade and besieged the Russian
Orthodox monks authorized to reside in Beijing.
The Qing had finished several victorious campaigns and extended their
territories and crushed vestigial Mongol resistance. They had built up
a skill set for war on the steppes that allowed them to win.
If they are sufficiently angry at the Russians they should be able to
mount an offensive gravely threatening Russian territories in eastern
and southern Siberia.
Qing territorial objectives, if Qianlong gets greedy, could include
seizure of the Buriat Mongols' lands, the silver-mining ditrict around
Nerchinsk, and a band of the fur-rich forest country north of
Manchuria.
At this time, the Russians are busy with the 7 Years War. Qianlong
could conduct his war on an entirely independent basis, but the
British could conceivably take an interest in the campaign.
Fighting the Russians would probably at least reveal they were
stronger than in the 1680s. It may or may not be sufficiently
disturbing for the Chinese to become interesting in cooperating with
the British in terms of purchasing certain types of arms and renting
naval assistance. Frederick in Prussia would be incapable of reacting
to the situation except simply to pray that Chinese attacks divert
Russian forces from Europe. The seed would be planted over the long
run in Prussian thinking of the potential of China as an ally. If a
true coalition war develops, Qianlong long may try, and succeed, in
having French missionaries and traders ousted from Vietnam. In the
long run, this might divert French colonial interests offshore to
Okinawa and Taiwan in the nineteenth century.
Scenario B) - Catherine the great gets the wild idea of recovering
territory (and glory) on the Amur, and gambles to establish a trading
empire along the lines of the emerging British Raj in northeastern
Asia. The occassion or provocation she can seize upon was the the
Qing's solicitation of Catherine's vassals, the Torghut Mongols, to
migrate from the Volga back to Mongolia/Dzungaria in 1771.
Russian Chinese cooperation was built in OTL on a degree of
expectation that the two sides would return each other's fugitives,
but the Qing dynasty as part of its strategy of in-gathering the
Mongols essentially stole potential taxpayers and soldiers from
Catherine, so this could be used as justification for a conflict in
the 1770s if Russia chooses on.
The near-term territorial objectives would be focused on seizing the
Amur and more of the Pacific coast to support Russian trade in the Far
East and North America. From 1774 onward, there is a break in the wars
with the Turks, and Suvorov could be available for service against the
Manchu banner armies. How would he do militarily?
At this point in time, 1775 and the years after, Russia has a fairly
peaceful environment in the west, and China has no one in Europe
likely interested in becoming a coalition partner, as the Atlantic
powers are abosrboed in the American revolutionary and colonial wars.
Actually, the Turks and Persians might be most interested in allying,
but they may or may not be capable of getting involved at this point.
Thoughts on A or B or other takes on the same overall theme of the
post?"