It depends on the circuitry in the card, some may Drive the LED from the internal 5V bus, some will drive it directly (via a resistor before the opto isolator i.e. on the input side. so as it is a QX10 then you cannot drive the LED with 24v or 5V (as some are) to test the input then you must apply the correct voltage for the card in the case of QX10 then it must be between 100-120v AC.
Forcing an input on will only force the image bits, the PLC copies the real inputs to an internal image before the main program scan (there are exceptions like forcing the real inputs to update the input image during scan).
likewise, the output image is copied to the real output image at the end of the main program scan (well in Mitsubishi anyway). This is common on many PLC's in other words the update of the I/O image is synchronous, there are some that use asynchronous, and most will allow forcing of I/O update by interrupt or during program scan.
Many in PLC's, forcing an input will cause it to be on for only one scan as the inputs are updated synchronously, however, there are some that have an enable bit associated with each input (or output) so that it can be forced on in the program regardless of the state of the real I/O or program controlling it.