Will cutting fat and salt help lymphedema?

Maybe. It might but usually this is an obstructive problem worsened by obesity, poor circulation and other issues.
Not really. It is not salt or fat that aggravates lymphedema. It is heat, inflammatory response, infection. The best lymph drainage therapy that works use chlorella drinks throughout the day with 4gm betaine hcl and 2gm modified citrus pectin twice daily 1 week on 3 week off to keep lymphatic system healthy.