IMO there is absolutely no reason to send Soderstrom back to Europe. He already has experience in a high level league over there. In fact, the only reason that I would put him in Tucson is to get him more experienced on the narrower sheet of ice. In every other way, he is ready for the NHL as a 2nd pair right now.
There are some reasons. One is the emotional development of a teenager being important. Also the fact that the SHL is a better league than the AHL and Brynas is a much better program than Tucson. He'll have access to a higher caliber of resources and competition there. That organization was already in the middle of developing this player extremely well, and he was on a great track there, anyhow. If he feels more comfortable there, and that's where he wants to be, it's also a really good option.
But Tucson is close to Phoenix and the big club can keep eyes on him and play an active role in his development here as he learns the North American way both on and off the ice, which should speed up the transition process that we would otherwise be pushing off another year. However, it might be better to push it off if he's not ready to be a teenager thousands of miles from home in a strange land. Sending him back to Sweden is certainly the safer option. And it's not a bad option. He'll improve and develop in a good league, for a good program.
If he's gung-ho to stay in North America and try to fight his way back up to the NHL from the AHL this season, and he thinks going home is a step backward, then I'd be happy to have him stay, too. I really believe Victor is the deciding factor on which option is best. I wouldn't want to make him feel forced into staying or rejected or stunted by being sent home. I'd really leave it mostly up to him.