Sweden's government can't give any assurances beforehand, since every case must be judged independently. Sweden can't extradite Assange anywhere after he's been extradited to Sweden from the UK without the UK's permission [0, 1]. If the US needs Sweden and the UK to cooperate, why wouldn't they just ask the UK from the start, who's a much closer allied?
[0] http://jtl.columbia.edu/wp-content/uploads/sites/4/2015/04/F...
[1] http://edition.cnn.com/2010/US/12/07/wikileaks.assange/index...