I assume they need to negotiate a sweetner here. I imagine the Padres want him in the deal more than the Nats though, so if it fell apart then there are many ways to salvage a deal. Hosmer is not much worse than Bell. And his contract is backloaded so he does have trade value (limited of course). I think the Padres wanted to get rid of that AAV more than anything else. He just needs to go to a team where the AAV is less important (i.e. a non luxury tax team). The Nats are unlikely to have luxury tax concerns the next 3 years so were an ideal place to dump the AAV and salary.
Edit - lots of reporting that Hosmer declining the trade would not kill the deal. It is a 10 team no trade clause so they probably can easily move him for little return, picking up as much salary as they need to make the deal happen.