Can mild internal (non-prolapsed) hemorrhoids heal on their own using conservative methods such as water and fiber? or does that just ease symptoms?

Yes to both.: Hemorrhoids are a part of normal anorectal anatomy. When they swell, protrude, itch and burn, conservative care is most frequently curative. Sitz baths with Epsom salts 3-6X daily for 15 min each over a 1-2 week period is most helpful. Prevention by avoiding straining at stools with fiber and increased oral water intake is helpful.