Is the presence of the suitor's family and their seeing the fiancée before the suitor a religious obligation or merely a custom, and what is the evidence for that? Is it true that if the suitor is unable to attend, his family may go to see the girl?

The suitor is not obligated to see the fiancée, rather it is recommended. It is permissible for the suitor to delegate a woman or a mahram man of the fiancée to look at her on his behalf. The evidence for this is that the Prophet, peace and blessings be upon him, "sent Umm Sulaym to a woman and said: 'Look at her ankles and smell her front teeth.'"