1&2) You do not program an amp sim using an editor. You use an editor to program the FCB to control the amp sim. You can use the FCB to control the amp sim in either standalone or VST mode. Assuming that you are using a PC (vs MAC), You can't run the editor and the amp sim at the same time due to conflicts with the MIDI driver.
3) No. You CAN run as many as you want (or as your computer can handle) using VSTs in a DAW or VST Host.
4) Yes, but only needs to be done for first use, then as necessary if you think that it's gotten out of calibration. Use a MIDI Monitor to check this. If the range is other than 0-127, it needs to be calibrated.
You use the editor to program the FCB according to the requirements of the amp sim and the way you plan to use it, then you close the editor, open the amp sim and assign the various pre-programmed pedals to control the amp sim's functions.
While there are similarities, S-Gear and Guitar Rig have very different ways of doing things. Using the Stock Chip with either will be very limiting.