AFAIK there is no way to pass parameters such as the originally requested URL to the server that shows the ad, so it wouldn’t know where to redirect the user after 5 seconds.
Maybe check the HTTP referrer just in case that contains it, then you could use any dynamic language to extract it.
No. To serve the ad the router takes the HTTP request from the browser and pretends to be the server that hosts that page and says “oh that moved here instead”. It’s impossible to make that happen in a new window or tab (servers have no concept of such things) as no JavaScript or links are involved.