I think you were very unlucky. I have just finished a 6 month stint living in Da Nang. I have to say it is one of the most scam free cities I have lived in, during my last 6 years living in Asia.
In complete contrast to your story I was only telling a friend yesterday how unbelievably straight forward it was leaving Da Nang airport. This is my story.
After paying the $25 stamping fee (Government fee, not a scam) I got a smile from the immigration officer and I nearly collapsed. They never smile, not unless I am giving a bribe at a land border.
After collecting my bags I declined the over priced SIM cards and thought I would take my chances with the airport WIFI. I found a weak signal next to door 1 on your map (above). The WIFI just about connected with the Grab server and my driver was on his way. Unfortunately I had no telephone number for him to call me on. Luckily it was a blazing hot Tuesday afternoon in the middle of June. The airport was dead, and the driver found me.
One thing that surprised me about Da Nang airport was that not one taxi driver bothered to walk up to me. maybe it was the heat.
As for Da Nang it is a great little city. The beach is great, there is a lot of construction. But I found a nice quiet apartment for $170 a month a short walk from town.
I found Da Nang to be a great city to hang out in and do some work. The locals are super friendly and I did not encounter one scam in my 6 months living there.
I am sorry to hear you had a problem at the airport. Da Nang is a great city and I will post a full report in the next few days.