If you don't mind the game going slow-motion when the fps drops, you can safely override timedelta with a fixed value. For puzzle games and such, that's fine - it's more a problem for action games, where everything going in slo-mo can give you an unfair advantage since you have more time to react, or the change in game speed would be annoying. That's what timedelta is aimed at solving.
Generally, though, I tend to say the best thing to do is enable timedelta, but not rely on pixel-perfection. That means don't make platformers with jumps that you can only make by a few pixels, or in your case, puzzles that can only be solved by a pixel-exact trajectory. Those are the things that might be affected by timedelta - and players might be less frustrated by your game if it's more tolerant, anyway