Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.
Multi-tracks are exported in a single sequence. To merge sequences...
Click the Sequences tab in the bin (upper left), select all but the main sequence, right-click on one of the selections and PLACE ON TIMELINE. (All sequences except the first will become single clips.)
An alternative...
Export multiple sequences. Click the MENU button (top-left), then FILE | EXPORT ALL SEQUENCES. It's a quick process.
Now each sequence is a clip. Import those clips, merge them and export again.