Is it possible to reverse an enlarged heart? what are causes of interstitial thickening? can it be reversed?

Sometimes yes, sometimes no; it depends entirely on the cause and available treatments. Discuss with your cardiologist. Good luck.

Yes. Treat the cause of the enlarged heart. Such as high blood pressure especially those associated with salt sensitivity. If this is the cause then heart will begin to get smaller immediately as BP comes down. So find cause and treat that.