We need a Paste Replace function really that works just by placing the cursor.
As it is now, you can only replace by pasting into a selection, and the selection must be the same length as the clipboard or the track length will change. You could try the pair of plug-ins “Punch Copy/ Paste” in https://forum.audacityteam.org/t/punch-copy-paste/28906/1 that don’t change the length when pasting.
Copy your 15 seconds of audio using Punch Copy/Paste, drag any selection longer than 15 seconds, then use Punch Copy/Paste to “Paste Once”. This will paste exactly your 15 seconds of audio, replacing only those 15 seconds. You can go to the Keyboard Preferences to set a shortcut for Punch Copy/Paste and/or Punch Paste.
So although you still have to drag a selection, you only need to make sure the selection exceeds the length you want to paste instead of it being exactly 15 seconds.
Gale