I think the important things for bumblefoot are a variety of surfaces, and
cleanliness. Last time I had a bumble rat, he had other health and mobility issues, so I had old towels, sheets, and those rag rug things as bedding. The most important thing is to keep it clean, so that it won't get into the bumbles.
The fleece hammock should be just fine.

As for the diet thing... putting a rat "on a diet" is a really hard thing to do! Hopefully their change in diet will get things back on track.