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.
How to disable the Cheatflag?
Use the NBT Editor, open the level.dat from this world.
In the folder data search for allowCommands. Change the Number to 0.
Is this world "playable"?
The way it is now? No chance. There are strongholds but no trees, no cows.
You have to optimize this world before you can realy play it.