Just wanted to chime in here... no puzzle on the site ever requires "What if?" or trial and error. There is always a pure logical solution to every puzzle. (Of course, sometimes it is faster/easier to just do trial and error when you get stuck, and of course there's nothing wrong with that!)
Anyhow, for this puzzle, here's a quick illustration showing one possible "pure logic" solution to the puzzle at what I feel is probably its most difficult point. In this case it required two advanced solving techniques used side-by-side in order to break the puzzle and result in a successful solve.
Hope this helps!