Is madi applicable for silk dhotis? Can a silk dhoti which is worn multiple times without being washed be considered to be madi? Or is it considered to be devoid of madi if washed?

Yes, madi applies to silk dhotis as well.
Silk is considered to be madi and there is no need to wet them. If washed, the degree of madi could become lesser but it can be considered a madi cloth.