It is a bitter lesson for me to learn something : always integrate an asset "after" testing with an empty scene and an empty project.
The thing is :
I had my 10+ tags, 10+ layer configs and a spiderweb of layer collision matrix and also special input mappings for my game. And just because i have imported this asset without hesitation means i have lost all of them now.
I know people are having hard time figuring out unity's project settings needed for the assets but i think you shouldn't have been added these in the asset. Maybe a seperate download of these settings would be better is it not?
And i know i should have backed them up with my asset exports

Another question: What is the maximum terrain resolution i can use with terrain generation script?