Spotlight / Show in Finder / in new Tab instead of new Window In a Spotlight search instead of clicking on the search-results, one may press ⌘ Cmd-R. This shows the file in Finder.

Problem: this always opens the result in a new Finder window (despite having set the general setting in Finder as Always open in new Tab).

Question: How does one force Spotlight to always open results in a new Tab of the (last opened) Finder window? I am looking for a Terminal command, or something which does not involve having to download a special programme. (I already installed Totalfinder and that achieved nothing.)
Same thing bothering me. As far as I can tell, there is no 'native' Finder solution. 

However, as a (belated) suggestion to OP, or anyone else with the same problem, here's how I solved it for myself -- sort of a 'hack', but better than Finder default behavior:

Use some keyboard customization software (I'm using Karabiner), and define a (user activated) shortcut that triggers two other keyboard shortcuts that are executed automatically, and in sequence. The shortcuts that do the trick are: (1) Reveal in Finder, (2) Merge all windows.

Details: "Reveal in Finder" is already bound to a shortcut natively (cmd-R), but "Merge all windows" is not afaik. Since adding menu items in Karabiner is a bit tricky, probably the easiest way is to assign a shortcut to "Merge..." in System Preferences -> Keyboard -> Shortcuts -> App shortcuts. I picked cmd-shift-M. You can then make use of this (OSX) shortcut in the definition of a Karabiner shortcut.

In Karabiner, I then defined a custom shortcut (cmd-R), triggering cmd-R and cmd-shift-M. 

The effect is pretty close to what I wanted to see in the first place: in a Finder search tab/window, I select an item, press cmd-R, which reveals the item in a new window, which is immediately merged into the existing window as a new tab. 

Focus remains on main window, the new tab goes to the end of the tab bar, and I can continue revealing other items immediately without having to cycle tabs.

The one thing that still bothers me: the "merge windows" animation is slow-ish, and I can't find a way to speed it up (or disable it).

Like I said, not perfect perhaps, but it's reasonably close to what I had in mind.