Entering accidentals

If you are using the computer keyboard the process is enter duration, enter note, press up or down arrow. No back to the entered note is necessary. If you have a key signature with an F# and you want a G-flat you will need to use the button on the note entry toolbar because the arrow will give you an F#. Another alternative is to press the J key if you have an F# and want a G-flat. All of these modifications of the notes can happen without having to press the back arrow. The next note button you press will put a note there and advance the cursor. On modifications, this includes such things as the tenuto line (shift-n) and any other articulation and ornament. It generally does not apply to items in the line palette but applies to adding grace notes - but you will have to advance the cursor after those.

Play around with it, it is really much easier than you make it sound.

If you are on the good Key, for example in Ab with 4 flats, if you enter a B, a E, a A, or a D MuseScore enter it according the key, it enters Bb, Eb, Ab, and Db, without doing nothing .
If you want to enter a note which is not "on the key" for example a Gb, with Ab in key, you click the G and in the same time the down arrow, and use the Up arrow in the same time you click the note for a Sharp

Assign keyboard shortcuts such as...

; double sharp
' flat
# sharp
= neutral

Enter duration using the numeric keypad, enter note by name, press appropriate button to add accidental if the note needs it.

Note that the original suggestion of pressing the flat key while typing "D" is still two steps, and it's basically the same as how you do it today, except you do it in the opposite order (D first, then flat), and you don't have to contort your hand or use two hands to do them simultaneously. So the current method is actually more efficient. But if you truly want a *one* step solution, try the Piano Keyboard window (see the View menu). MuseScore will have to guess if you want it spelled Db or C#, but if it guesses wrong, it's still only one more step to fix it - press "J".