3) To the best of my knowledge, SMPS can not be truly variable, as a linear supply. Minor adjustment is possible, however.

Yes, they can. For instance, with a configuration known as a SEPIC converter, you can generate output voltages that can be varied over a wide range and can be lower and higher than the input voltage.

The thing is of course, that the device has to be specifically designed to be variable. This is definitely not the case for a PC power supply. However, I think it would be possible to 'crank up' the output voltage to about 7V, say.

Best regards,
Rob.