Is a 7-minute transfer (from an ICE train to a regional train) too risky? If I can't make it, can I use my ticket to take the next regional train?
Any help will be appreciated!
Yes, 7 minutes is pretty risky on DB, although when coming from an ICE and changing onto a regional train, chances are higher that the regional train will wait for passengers from the ICE (compared to an ICE waiting for pax from a regional train, however, without knowing the station, scheduled platforms and details of the specific trains, it's impossible to give an absolute estimate).
Still, I'd suggest you make sure that arriving on the next train won't hurt your further plans.
As for your second question, yes, you can simply take the next train, there is no "Zugbindung" on regional trains, so you can always buy a ticket for the short connection and take the next train if the ICE is running late.
The Nuernberg station only has one level with tracks, but you need to go downstairs from the platform to get to the others, and the walk is long if you have luggage.
If the train is on time, you can make the distance between the first and the last platform easily in two to three minutes if you know the station. Essentially you find the nearest stairs, go down, look left and right to see the number of the platform you are looking for. They are numbered in ascending order, with large blue signs next to the stairs.
When you go downstairs from your platform where you arrive, there are already signs that show what way you need to turn.
I believe even with luggage, 7 minutes is enough to navigate this with signs. If your train is late however, you should inform the staff on your previous leg that you have to make the connection at least ten to 15 minutes before arriving in Nuernberg. They will either make the other train wait, or find an alternative connection for you.