My view:
1) If no stiffness value is entered for a translational boundary condition, the program will apply the default value set in the configuration file. This will be those you list and also GUIDE, ANC, the bilinear group (e.g., X2) and the rod restraints (e.g., +YROD). This also applies to entered translational displacements where no explicit restraint is defined.
2) The cell remains blank. Be sure to include the configuration file as part of your archival documentation.
3) More accurate analysis is better but stiffer is more conservative when evaluating strain. There are trade offs...