Agreed with many of your points. However, there is one assumption that we have not made. All the old Ming Generals that are left over after Li Zhicheng took over Beijing are in control of their own troops.
There are a few things that they could do,
1) Lay down arms (as you have suggested).
2) Fight Li Zhicheng for various reasons - such as avenging the old dynasty, fight to control China so that they themselves can be king.
The second was more apparent for Wu Sangui. He knew that he couldn't defeat Li Zhicheng alone, he needed the Qing Army's help. Thus he opened the gate and let in the Manchurian... and in actual fact, he was made king in control of a vast area - Yunnan...
These are all valid points. I don't doubt any one of them. I think my point is that if Wu kept his defense intact, China would have had more options. With these potential options, the victory of Qing would not be that destined.
I actually think that the situation at the end of Min dynasty would be comparable to the time before WWII in China. In the 1930's, there were many factions and local warlords in China and the country was seriously divided. In fact, China had been in a nation-wide civil war for 70-80 years when Japan invaded China in 1937 (from Opium war, Taiping, invasion of eight nations, end of Qing, northern expedition, etc). So in terms of the conditions in China, I don't think it would be too much to say that it's pretty similar to what was like at the end of Ming. In 1930's, although KMT's Jiang was officially considered as the leader of China, majority of China was definitely not under his command. Many warlords were battling it out to gain power. This of course include CCP. Many of these factions, including CCP and KMT, were still battling each other during the fight against the Japanese. So the chaotic situation was not unlike the end of Ming.
And I am almost 100% sure that the gap between Japan's military strength and that of China's in 1930's was a lot bigger than the gap between Manchuria and China at the end of Ming dynaty. Yet, the strategy of space vs time worked in WWII. It shows that in a nation as big as China, so long as any form of govn't stays intact, it has a chance against a foreign invasion. So it would not be surprising at all that the same strategy would also work at the end of Ming dynasty.
We can talk about all the possibilities for or against China's potential chance to survive Qing's invasion. We cannot deny that China would have a lot more options if Wu stayed loyal to China. Any of those options may help China stay independent and push Manchurians out.
Oh, one more point. It has been suggested that Manchurians could easily enter China from somewhere else if Wu did not open the Pass. Well, Manchurians had been courting Wu for a long time, even promising him the chance to become the new emperor of China at one point of time. The fact that they didn't bother bribing other Ming generals in other locations suggest that Shanhai Pass is actually strategically important and other entrances may not be as useful. So either it was legistically impossible to get into China via other entrances or they would have had to go through many more obstacles if the chose other Passes. So that definitely suggests the strategic importance of the Shanhai Pass. And by giving up such important strategic location, Wu definitelt bears the most of the blame for the fall of China.
Also, I believe that saying "Manchurians could've done it via other ways" sounds a lot like shifting responsibility. It's like someone is caught shop-lifting. The thief might say "well, the security of that store is so low, someone will steal those clothes sooner or later even if I don't steal it..." that does not change the fact that he/she is a thief. Excuses, excuses, excuses...
So stop giving him excuses. Call him for what he is, a traitor.