Hi LineChaser,
The only one of your questions I can answer is that there's no direct way you can make them backwards compatible. I have a couple of suggestions. One is that you might be able to import the file from the newest version into
GPSBabel or
GPS Utility and export it back out into the format of an older version. Eg. You would do something like import the .est file, export it to some other format like .GPX, then import the .GPX file and export it back to .est.
I have worked with the two utilities only a tiny bit so I don't know how well you can control their output format when exporting to Streets&Trips format. I also don't know whether you will lose anything in the process, depending on which interim format you choose (the chosen interim format, eg. .GPX, might not support all features of the .est file).
Second, I recommend that if it's important to you to be able to share your work with the other two people that you get them to upgrade to the same version as you. The Streets&Trips version without the GPS receiver is cheap like dirt. Failing that, and still assuming it's important to share with them, you could consider using the oldest version yourself. Generally, the newer versions can read the older files but not vice versa.
...ken...