How to change the seed?
Download an NBT editor from the internet, open the level.dat from this world.
In the folder data you can find the RandomSeed. Just change the number.
I suggest you do that before you load the world, otherwise there will be chunks generated you would have to delete afterwords.
If you have messed up that step just download the world again. It's shorter this way and i dont need to explain the other.
Are there Villages / Strongholds?
Yes! Villages i have seen. Villagers themself not jet. I think they just fall of into the void if the village is generated in the air.
You can cure zombievillagers. Follow this link: https://minecraft.gamepedia.com/Zombie_Villager
Why are there barely any animals?
That is becouse (i think) there are so many dark places underground where monsters and bats can spawn. The mob spawn algorithm checks the lower places in chunks first.
To force animals to spawn lit up all caves you can find or build a grassblock area very high up, where nothing else can spawn. If you build up the area very high it will still take some time becouse the algorithm checks from bottom to top.