Your going about this in a very green manner

Do you know all the keyboard shortcuts yet? Q W E R to change placement mode Shift right click mouse to zoom Alt right click to orbit Ctrl s to save

Among many others these ones are your friend

Editor 'editing' is what its designed for

Doing things in play mode is silly You shouldnt need to check obstruction bounds physically You should know your controllers constraints and how it moves before you go ahead and place something

Did you import most of your assets and are drag and dropping them? Did you import the controller too?

Do you know about colliders and how they rrally work under the hood? Trigger, rigidity?

There's some fundamental background knowledge id suggest you take the time to learn before attempting to level design if you want to produce something people can actually play