の here is the possessive marker, not the nominalizer; ご is an honorific.
かな adds the sense of wondering.
I wonder whether is that the arrival of the representative.
代理人 must be someone important or esteemed or who should be respected, thus the use of the honorific: ご到着
Without further context, it's a bit more difficult to say much more.
代理人のが到着かな doesn't make any sense grammatically. If you wanted to make
代理人 the subject of a verb, they could have said
I wonder did the representative arrive?
You can't use the honorific prefix here.
I'm not entirely sure how to form a suitable honorific here. I'm used to hearing
But that would not be suitable here since いたす is humble. (You hear this expression over the loud speaker in the train station for example.)
I suppose it might be
Some feedback on this last point might be nice. Keigo is not my forte. I understand it usually when I hear it (assuming it's not over the top), but it's passive knowledge, not active for me.