Working with Excel: Does GuiXT allow me to read data from - and write data into - specific cells in Excel files, eg by using an InputScript or VBScript? In the documentation I only learnt how to write data to Excel files in sequential order, not to specific cells.

This is possible with VBScript. You can use ApplyGUIScript or the GuiXT Controls component statement CallVBS.
We recently recorded a short video about Excel integration. The recording is in German but the images show the GuiXT script and VBScript statements that you need: