1) Not possible. Maybe it is time to redo the “Flight statistics” dialog box layout. Not sure that many use the detailed information. I could add a box to tick to omit the information in the .kml file.
2) A GPS product information text string is always included in the “FsInfo” tag. This does not always include a serial number.
3) The track gap time is used when you initiate a save of selected track log points. GpsDump scans the track to see if there are gaps that are larger than the value you have set. If there are GpsDump will present you with a list of possible tracks. Some older instruments (e.g. Log_It) just had one big track.