Very well thought out post Jesse. I totally agree with everything you said.
I think in addition to be outnumbered and surrounded, the Empire vastly underestimated the Ewoks. And that is one of the biggest mistakes any military commander can make.
Going back to Jesse's example of Little Big Horn, Custer and the 7th Cav. were not only outmanned and surrounded, but they underestimated the Native Americans. I'll give some of you guys a little refresher on Little Big Horn, in case your history is a bit rusty.

The Native Americans had been put onto their reservation in the Dakotas a few years prior to the battle of Little Big horn. White settlers began moving into the land in the Dakotas that was supposedly for the Natives. The Sioux took offense to that and under the leadership of Crazy Horse and Sitting Bull, a large army of warriors, about 2500, left the reservation. The US Government told them to go back but the Sioux refused. Custer and the 7th Cav. were sent in to drive them back by force. And here is where they really screwed up. Custer and the US brass thought that the Natives would be scared of the army, and would return to the reservation as soon as they saw them. And they thought if they did have to fight they had a big technological advantage with their weapons (which they did) and as soon as they started firing the Natives would run away in fear. They of course didn't and they all stayed and fought killing Custer and all of his men.
And there are countless other examples in which someone has underestimated their enemy and get their ass kicked as a result. Like Japan, they expected us to give up after Pearl Harbor. They though it would completely discourage us as a Nation and take away our will to fight. Of course it ha the opposite effect and launched us into WWII. After WWII the US was the only remaining world power that hadn't been devastated by war. We became the world's Superpower. The Soviet Union quickly became the only other superpower, because Russians are crazy like that. And Through out the world the smaller nations began to ally with either the US or the USSR. Then came Vietnam. The Vietnamese had been a colony of France for years and in WWII fought on the side of the Allies against Japan. After the war they wanted their independence from France, who was reluctant to give it them. Then after several years of fighting the French pulled out (go figure

) and the US sent in military advisors. The Vietnamese were leaning towards communism as many developing nations did, because communism was more efficient and was easier for a developing state than democracy. The US though that if they sent in their forces they would easily be able to subdue the outgunned Vietnamese who were tired from years of war. They were very wrong. The Vietnamese, many of whom were still using WWII era weapons, eventually drove the US out of Vietnam. Their guerilla tactics were instrumental in their victory.
There are many parallels to the Empire vs. Ewoks that can be drawn from this. Like the Vietnamese, the Ewoks were expert guerilla warriors. The Empire thought like the US, at Little Big Horn, that the Ewoks would be scared when the saw the Stormtroopers and would run as soon as they were fired upon. The Empire also felt that their technological advances over the Ewoks made them invincible and as a result saw the Ewoks primitive and didn't feel they were a threat.
As a result I think it is very plausible that the Ewoks beat the Empire. When you take in to account what Jesse said, the Ewok's weapons didn't have to penetrate they armor. All they had to do was break bones through the thin armor. The ewoks were able to down several AT-STs because they were able to find the weaknesses of the walker, its balance and it's relatively thin armor. I'm blabbering though, so I'll stop.