I don't understand either tbh. Like you said, even back then Insomnia was a war zone and not a place we could explore peacefully at our leisure. It's an interesting looking city for sure, but it doesn't fit into the main narrative like you mentioned. The only route is making Insomnia explorable as extra content, not a part of the main narrative before the end game. Adding it into the main narrative would lessen the impact of the player arriving in Insomnia for the first time after 10 years.