This functionality is not present in Fantasy Grounds as of version 3.3.9.
The only "discard the lowest" functionality that exists in the FG rule set 5e is to launch a d20 with advantage.
However, there are some third-party extensions that add this functionality.
Nor is there a "repeat last entry" command. However, you can create a shortcut in one of the quick slots for any chat console command.
Source: My extensive experience with Fantasy Grounds.