I think you can avoid going through immigration, though if you are to spend time in Arlanda (and don't need a visa for schengen) I would suggest that you do. The lounges, and most of the shopping and food outlets are in the schengen area. My experience with going through immigration and transit seurity in Arlanda is limited but generally it has been very quick. It would most likely with 2 times immigration (in and out of schengen) and security leave you with 1,5 hours to spend in the airport. And they will be more enjoyable in the schengen area.