String Theory is a connection and untangling puzzle: strings cross each other, and the job is to rearrange the points until no line overlaps another.
It's a planar-graph problem in a friendly wrapper. Every solvable level has a crossing-free arrangement, and finding it is a matter of moving one point at a time rather than trying to see the whole solution at once.
Spread the points apart first, then fix one crossing at a time. Trying to see the whole solution at once is much harder than resolving it incrementally.
Yes — each has an arrangement where no strings cross, which is what makes it a logic puzzle rather than a guess.
Yes — drag points with a finger. Nothing to install.