I would "populate text field" in the UI in the top left:

Send keys for the cell you want (the left cell) and Enter...example: A5{Enter}
Send keys for {Shift}({Right}) to simulate Shift + Right Arrow

Send Keys for Merge Cells which is {Alt}()hmc NOTE: the excel hotkey menu is slower than PAD, so change the delay from 10 per keystroke to 100

Alt hmc gets merge and center, you can replace the c with a or m for different actions like below.

Best of luck!