Although I would generally consider the two words synonymous, I believe that, by etymology, "doofus" implies a more "goofy" or careless act of stupidity. The word "idiot", by less familiar definition, is used to describe a mentally incompetent person and therefore, in its more common definition, should mean a person who is simply stupid.

The sentences is ambigous so both sentences are possible as written.

1) He argued smoking is bad for you.
2) He argued smoking was bad for you.

Both are correctly written,
But #1 means smoking is bad for you and still is.
While #2 means smoking was bad for you in the past, and this may or may not still be true.

I'm not sure what the actual rule is but it is actually dependent on what you are describing.
For example to say "the dog was on the roof" is appropriate, but then again you wouldn't say "the dog was on the basement" would you?

Both are houses, right?

Same with transportation, you can get ON the train, ON or IN the bus, or ON or IN the plane, but not ON the car.