Generally speaking, a screw up here and there shouldn't hurt.
Its consistent, dramatic changes in environment and constant light leaks that cause big big problems.
Everybody has a timer issue here and there nothing to worry about. But yeah, don't rely on other people to take care of your stuff as well as you would.

On the light penetration issue, don't do a big cut. Your plant works hard to create those leaves and removing them is just not good for the plant. Also, when you remove 15% or more of the leaves, that causes a HUGE stress on the plant. Tie the branches off and move them around to allow for better penetration