Thank you, you were right about the eraser error! Both eraser and pen now work independently when the pen is inactivated as described in the previous posts. But Gimp keeps to be "confused" when the tools are deactivated:
This is how a normal Gimp (tools activated) looks in the upright corner: Activated.png, the first one left
And this is how it looks when the tools are deactivated: Deactivated,png, the second one left.
Obviously, in the latter case I loose the ability to modify the size of the Gimp window
because this little square between "-" and "x" is missing. Well, seems I cannot have both worlds 
P.S.I did not check what other features of Gimp are affected by the deactivations.
This is how a normal Gimp (tools activated) looks in the upright corner: Activated.png, the first one left
And this is how it looks when the tools are deactivated: Deactivated,png, the second one left.
Obviously, in the latter case I loose the ability to modify the size of the Gimp window
because this little square between "-" and "x" is missing. Well, seems I cannot have both worlds P.S.I did not check what other features of Gimp are affected by the deactivations.