The question is, how do you explain the phonological and semantic differences between *saiwalo and solus? The modern reflexes are homophonous, and it's easy to see how they could be semantically related, but I think the fact that they're less similar the further back you go is a sign that they're not related.
If etymology is like evolution, then I think of words like these as sharks and dolphins. They look a lot alike superficially, but it's really just coincidence.