I can see how a lot of your problems are caused by the new game mechanics in the recent snapshots. Singleplayer is now multiplayer, it's basically a LAN server. This is very pointless in my opinion, but I believe there is a reason, whatever it may be. Now your game is running a LAN listen server and a client, which may amplify performance issues.
Also since the snapshot, animals and blocks will randomly freeze for
an unspecified amount of time and I can run around but not interact
with anything.
This specifically is because of the new multiplayer-singleplayer thing.
I would suggest going back to the recent version of the game, not the snapshots. If the snapshots ever become the full game, you may have a problem. Not to sound mean here, but, a lot of people foolishly think all performance issues involving games can be fixed. This is not true. If you get the chance, get a new computer. That's the only real definite solution to performance issues.
Also, as neosatan's answer mentions don't use mods or texture packs. That will definitely slow down your game.
I usually don't answer my own question, but in this case I have to I guess.
I am running Minecraft pre-release 12w36a and I can confirm that you can cure infected villagers spawned from dungeon zombie spawners in survival mode.
I had a zombie spawner underneath my base that I never destroyed, I am glad I kept it. I set up a small killing device for killing the unwanted zombies, and waited for a infected villager. In my case it spawned one every 30 zombies or so.
When I got one I used a splash potion of weakness (water bottle + fermented spider eye + gunpowder). As soon as I threw it I got my golden apple in my hand, and just right-clicked the villager. At this point he started shaking.
While he was shaking, I waited around. About 5 minutes later he turned into a normal villager.
At this point I wanted to know if he would despawn, so I went exploring in my world, slept for a few days, and went to the nether a few times. When I came back he was still standing there, which was great.
I do not know if this will stay in the game. I will update the game once in a while to see if it still works in the snapshots.
For now I will work on setting up my village, I am glad I don't have to build the 1000+ block railway anymore.
I can confirm this works in 1.4.2, see the video in the question for more details.
Best Answer
Zombie villagers are a hostile mob that make up 5% of zombies that spawn in the Overworld. They will spawn just like normal zombies. You can capture and heal them to get villagers at your house if you would like.
Quote from the wiki: