To fully answer your questions I'd need to know what format you are exporting the image to (JPG, TIFF, etc.)?
Regarding your question 2:
When you import an image into lightroom the image is saved (in the folder that you specify) and any manipulations are contained in your catalog. If you export a file after adjusting the image the imported image remains in the folder that you imported it to. Therefore, when you return you should find the image in your catalog with the history of your prior adjustments and you can continue to adjust. Lightroom does not alter your original imported file; the catalog saves a record of your adjustments so that you can always go back or continue forward.