This is a very handy script!  :)

Here are two additions I found useful. Add them if you like them.

1. Add box for typing or pasting a path from clipboard when browsing for root folder:
change
FileSelectFolder,startfolder
to
FileSelectFolder,startfolder,,3

2. When starting the program, populate the ListView with the folder delempty.exe resides in and all its subfolders.
change
includefolders=0
Return
to
includefolders=0
startfolder=%A_ScriptDir%
Gosub, COUNT
Return

Here's a request:
When selecting multiple rows in the listview, allow to toggle check/uncheck checkbox for all of them with space (currently only one row gets checked/unchecked). I don't know how to implement that in AutoHotKey myself, but I bet it can be done.

the most important part of this idea is that the special key comes as a PREFIX to an action/modifier

Agreed.

But is the combination of "initial space prefix" plus some other suffix/prefix for multiple modifiers out of the question? Just try to type " mp3" and compare it to "+mp3". The former is a lot quicker (due to space bar size and position). Come to think of it, the most coherent way to have the "initial space prefix" might be to just make it an (optional) addition on top of another prefix.

Example: " mp3 +edit +d: moby" would (i) search for mp3-files, (ii) set action to edit, (iii) narrow search to D: and look for files with "moby" in name.

Those who don't like the "initial space prefix" could still type "+mp3 +edit +d: moby"

Another quick "extra prefix" would be to read all phrases that begin with an uppercase letter as modifiers ("Kill" is a modifier, "kill" is not).

re: actions and modifiers:
1 & 2! Also, since most modifier phrases will be brief (mp3, edit, kill...), allow multiple columns in the result list (edit: only for the list of actions, not for regular results). That gives maximum overview of available actions and speeds up selection.

Mouser,

Ok, I see the problem now.

"and let users have two keyboards so they can type in both at the same time and restrict results in both windows."

Let them have four hands too  ;D

One could perhaps handle the problem with FARR interpreting misspelled action modifiers as regular search terms by demanding some unusual prefix letter before all actions. Alternatively, when pressing some hotkey, FARR interprets the next term as an ACTION specification (and highlights the term to show that the hotkey worked)
A downside is that users must press an extra letter/hotkey.

With risk of repetition, another solution is to sidestep the problem more generally by letting users specify actions and/or filters for file-types or certain folders through hotkeys alone. (Yep, I'm turning more and more into a hotkey fanatic)

GOOD THINGS WITH HOTKEYS:
- Hotkeys have less ambiguities - either you press the right one or you don't.
- When known, hotkeys are faster than typing.

BAD THINGS WITH HOTKEYS:
- Users may fail to memorize them, making them slower AND frustrating.

But that can be handled by some hotkey help displayed in the GUI, like the L i illustrated or like how Adobe Photoshop shows alternative actions in the taskbar when for example CTRL is held down.

- There's tension between uncomplicated hotkeys and having many alternative actions/filters to choose from. Handling the latter through hotkeys necessitates more complexity. Which take more time and more effort to remember.

But I think a good trade-off would be to
(1) to (as said before) guide the hotkeys through help in the GUI and
(2) limit them to one- or two-key-combos. For example: simple hotkeys = one of CTRL/ALT/SHIFT. More complex hotkeys = simple hotkey + another key (perhaps right SHIFT can be used for that?). That give's six modes in addition to default. Wouldn't that cover most usage for most users?  :tellme: The rest could be handled via context menu.

Also, if the hotkeys are given different usage for when the search field is in focus (call them search modifying hotkeys) and results list is in focus (call them action specifying hotkeys) we get six file/folder filter modes and six action modes from only four keys (in addition to the default modes).