How to Navigate within bash's Reverse Search?

You can access this via the forward-search-history function which is bind per default to ctrl+s. Unfortunately ctrl+s is used to signal xoff per default which means you can't use it to change the direction of the search. There are two solutions for solving the problem, one disabling sending the xoff/xon signaling and the other change the keybinding for forward-search-history

Disable xon/xoff

Run stty -ixon in your terminal or add it to your ~/.bashrc. This allows you to use ctrl+s to use the forward-search-history history function.

For more information about control flow have a look at How to unfreeze after accidentally pressing Ctrl-S in a terminal? and some of the answers

Change the keybinding

If you don't want to change the default behavior of ctrl+s you can change the keybinding for forward-search-history with bind. As most keys are already defined in bash you may have to get creative:

bind "\C-t":forward-search-history

This will bind ctrl+t to forward-search-history, but please be aware that per default ctrl+t runs transpose-chars