Numbers are already written that can't be exceeded.  

There are already well defined numbers in Part 135.

14 hrs of rest in a 24/hr lookback is a hard number.   You cannot plan to exceed it!  Simple.

Rest is ANY work performed by the company!   Well defined by the Chief Council; just need it in definitions!   Far too many attempt to use the FLIGHT ATTENDANT definition rule (135.273) for pilots.   Not applicable!

If you WORK for the company, it is not a "day off".   If you go to training/ground school, it is not a "day off".   If you travel for company, it is not a "day off".   It is duty and needs to be shown on the duty/time logs (accounted for).

You are not limited by 14 hrs of duty, you just can't plan to fly Part 135 flights past that point without seeing 10 of rest in the preceeding 24 hrs.   You can work (be on duty) indefinately after the flying ends.

Part 91 is Part 91!   If they are going to apply duty restrictions to Part 91, then they need impose same restrictions to every Part 91 operation, including all general aviation operations.

There is nothing broken with the current Part 135 duty/rest rules except nobody seems to know them or properly operate under them!