The only Robert Douglas I found on the IGI (see Familysearch.org) who was bapt in Douglas around that time with father Robert Douglas is: ROBT DOUGLAS Christening: 14 OCT 1828 Saint Mathews, Douglas, Isle Of Man, Father: ROBT DOUGLAS, Mother: MARY WILLIAMSON.
On LDS film no.0106718

I can't see his father's burial in 1864, or a marriage to Mary McCaskie. Are you sure they were in the IoM? You can check bapts and marriages for yourself on the IGI. (See extracted records [from par reg], not unreliable submissions from church members.)

This is Brian Lawson's burial index, which has Douglas burials (includes surname Douglas):
www.iomfhs.im/lawsons/LawsonBMD/burials/dz_do_dro.html?Submit=+++Doak+-+Drover+++

His index to wills:
www.iomfhs.im/lawsons/LawsonBMD/wills/d-will.html

You need to read his explanatory notes and the tops of the columns, but these include the LDS film number for each will. They can be viewed at local Family History Centres for a small fee. (Also the parish register copies on film.) See in phone book under "Church of Jesus Christ of Latter-Day Saints" - open to all.

I can't see William L, son of William. Have you looked at the actual death certs, or just at the NSW index? The death certs are often very informative, giving parents' names and addresses as well as a lot of other details not available in UK/IoM records.

Sue