I readily admit I haven't tried Behlen's varnish. It's not that a varnish should be impervious to salt water- I think any varnish should be unaffected by either salt or fresh water- it's the amount of UV shields in the varnish and its adherence to the hull and its flexibility that are important. Of a variety of varnishes tried, personally I much prefer Epifanes Gloss Varnish. It's available from good marine suppliers including Jamestown Distributors (search online; you'll find they're very helpful with a variety of finishing materials and supplies, almost any fastener you could want and many other things).
About the spreading- you might need to add some stiffening battens outside the hull where the outwales would go (no nails or screws, just clamp on), and gently, maybe slowly over time pull the hull back into shape. If you simply pull it together quickly at the thwarts, you could break some wood, or you might end up with the canoe pulled in more at the thwarts and bulged out between them. Old Town's 15' 50-pound model often develops a peanut shape on its own because it has only one thwart. Between that thwart and each end, the hull can relax. With the battens for now, and after getting outwales back on, yours shouldn't have this problem.
The canoe is looking good- keep it up!