I wouldn’t say that is “well used” in the context of other stations in the area. I know the area very well, and closing the station would simply cause most of the passengers to use WGC or Knebworth instead. Some passengers would, of course, switch to other modes.
I also know the area very well.
It is hilly, hence a 100 feet high viaduct and 2 tunnels. The local roads are just country lanes with steep gradients not very suitable for buses, or walking and cycling.
Welwyn North survives because neither Welwyn Garden City or Knebworth are suitable alternatives. For Welwyn North users switching to Welwyn Garden City or Knebworth is not simple. Although Welwyn Garden City is less than 2 miles away, it is the other side of the 100 feet deep river valley and has no parking (apart from the shopping centre). Knebworth is 3 miles away, the other side of the 2 tunnels, and only has a very small car park. Closing Welwyn North station would make life very difficult for the people who live near the station and use the trains.
Is there a case for making Welwyn North station 4-track, to allow Intercity trains to overtake?
No. The opportunities to pass are either side of the 2 track section, at Digswell and Woolmer Green.
There isn't room to make the station 4 tracks because it is a very short distance between the viaduct and the tunnel. I doubt that turnouts faster than 20 mph could be fitted in, which would make the stopping trains even slower than they are now, and use up more line capacity.
And then there is the amount of disruption that would be required to achieve a negative result.
Anyway, the line was provided with 4 tracks a century ago when the Hertford loop was built. With only a 2 tph stopping passenger service and a few freight trains there is some spare capacity there.
There is also still spare capacity while paths are still being taken up by 5 car trains that could be twice as long.
Another important constraint to be borne in mind is platform capacity at Kings Cross. It is no use running more trains through Welwyn if there is nowhere to terminate them when they get to London.