I am very glad that the checkbox mode is now available for all folders at once. However, I would like to add one more option in the settings so that the checkbox mode would behave the same way as in explorer - simultaneous check-select action avoid preview in Viewer Pane depending on whether you clicking the checkbox or the file name
Now I have to toggle check-select files with help of the keyboard, and I can't use the middle mouse button, because it already assigned to the Enter command.
In other words, let there be an Explorer checkbox mode as an additional option.
What's wrong? Few years ago I saw that some people missing MS Explorer checkbox mode, why now not? Is this a difficult decision or not important at all? More options, more flexible.
We dont intend to make any changes to checkbox mode sorry.
You mean simply turning on "Checkboxies to Selection" or "Selection to Checkboxies" with check mark require big changes? It will just be an instant action, no need go to menu every time.
I don't know if this helps you, but I discovered here on the forum that you can select files with the middle mouse button (scroll), without the need for the selection box.
Try it out and see if this works better for you!
Exactly this.
Checkbox Mode in Opus predates File Explorer having checkboxes at all, and the whole purpose of it is to let you build up a selection of checked files to do something to, while still being able to click on files to display them in the viewer pane without losing your selection.
If you just want a way to toggle-select individual files without having to hold Ctrl, you can do that in Opus using the middle mouse button, without adding checkboxes everywhere that are fiddly to click on.
(You can also make the left mouse button do the same if you want, but only in Power Mode, whose purpose is allowing for non-standard mouse actions.)
As I mentioned earlier, I have my middle mouse button press reassigned and don't going to roll back. If you don't want to change anything in checkbox mode, then just add an option to combine "Checkboxies to Selection" and "Selection to Checkboxies" in one action. Then choosing combined mode will avoid additional moves when clicking a checkbox or file name. This option can be instead of pressing Ctrl and will become similar to File Explorer 
I'm a bit confused what exactly the expected behavior is for you. The only real difference I could find with Windows Explorer is that Windows deselects/forgets the checkboxes whereas Opus remembers the checks upon toggling. Neither turns checkboxes into selections or vice-versa for me.
I assume checkboxes are scriptable like anything else, so it shouldn't be too difficult to turn a selection into checkboxes or the other way around without the devs implementing it.
I know it and at this time using keyboard shortcut for this. I miss only additional Windows Explorer mode and can't understand why developers hate it 
What is this "additional Windows Explorer mode" you're talking about? Maybe (or probably, by the looks of it) it's just me, but I'm getting more confused the more I read what you posted and elaborated on.
Are you sure you can't fix or automated it with a button/script? Given you said it would be easy for devs to implement, I can only assume that should be easy too.
Unfortunately, the discussion has reached a dead end. I have nothing against the current checkbox mode, let it be as is. I just can’t understand why the Explorer option is bad and why it can’t be added?