Is the concept of continent questionable?

Sure. Continents have been conceptualized by convention only. It is accepted that Australia is a continent, but that Greenland is an island. (Greenland has been determined to be a part of the North American continent, a shelf-like extension, so to speak.) However, if Greenland were in the middle of the Pacific Ocean, for example, it could easily be conceived to qualify as a continent.

Sure. Continents have been conceptualized by convention only. It is accepted that Australia is a continent, but that Greenland is an island. (Greenland has been determined to be a part of the North American continent, a shelf-like extension, so to speak.) However, if Greenland were in the middle of the Pacific Ocean, for example, it could easily be conceived to qualify as a continent.