The uslessness of many websites is overwhelming.

The most overwhelming was the stone

In some instances, yes. If something is overwhelming me it is preforming the action on me- If it is an action it is a verb.

If I am overwhelmed I am being described as being overwhelmed so it is an adjective.

It can probably being adverb too if a action or another adjective was overwhelming. "her overwhelming speed."

overwhelming, overwhelming amount, too many

An overwhelming sound is a sound that is too loud or piercing for the average hearing of an ear.