Is the Hajj valid if one stays in a tent in Arafat all day and does not ascend Mount Arafat, and are the tents designated for campaigns in Arafat considered a valid standing place that suffices for staying in?

Standing on the mountain is not a condition for the standing at Arafat. In fact, ascending it is not from the guidance of the Prophet (peace be upon him). Arafat is not restricted to the mountain area; it is larger than that, and there are signs indicating its boundaries. So, if you remained in the tent, you have fulfilled your duty, and your Hajj is valid.