Thank you so much for your feedback!
The gem pickup issue is something we are already planning to fix after the jam. It was definitely a slight oversight when we were putting things together in the end.
To get the higher scores you have to utilize the rewind mechanic pretty heavily. You can trigger it manually (which there are prompts for in the top right and it is taught in the beginning) That way you can move around and collect stuff, and rewind and get all your time back while also keeping all the progress you have made.
There are ways to exploit that mechanic but we already are working on thoughts to fix that after the jam.
Glad you enjoyed overall!
-Gruntled