is there any information which would not appear in a narrow route briefing but which would appear in other types of briefing?
Yes, info whose radius of influence does not cut your narrow route (well you asked for it) If you want the whole thing go for a Route Briefing.
20 miles beyond both airfield would reduce the risk of hitting a tra or other restriction if you accidentally go beyond destination.
If your nav is that bad you ain't going to be able to avoid the TRA anway!
TRA's generally have a radius of influence sufficient to appear if you are flying near them rather than into them.
Let's see, 20 miles beyond both airfields x 20nm route width, that's another 800 square miles!