question about the landscape

I imported a landscape into my project which is made by world machine , and all goes fine before i accident deleted the landscape.
after i used ctrl+z to undo the delete command , the landscape came out , but whenever i want to put something on it , it will just pass through the landscape
and locate on the object (my static mesh used for the sea) behind my landscape , does anyone know how to fix this problem? thanks a lot!

p.s. i already tried building it again

and i got i few more questions here , so all the questions is :

Q1: how can i fix the problem that i cant let the objects locate on my landscape right. (i could before i accident deleted it)

Q2: dont know what i did that whenever i selected an object , theres a red cross right on it , how can i turn it off?

Q3: The reflection on the water is opposite from the real world , is it still unfixable?

Q4: can anyone show me how to let the skylight become darker while i used the simple daynight cycle rotating the sunlight(directional light) and the skyshere?

Q5: can anyone show me how to let the fog density change randomly in time? ?

Thanks so much for anyone who answers me !

(this is a picture that maybe can help u understand what i’m talking about )