https://familysearch.org/pal:/MM9.3.1/TH-1-16946-74142-31?cc=1849782 - two interesting things looking at the original of the shipping list
1) Some others on the list are mentioned as deceased - dates of death are written in on the final column. This suggests that William did make it to the US.
2) William is listed above Abraham, not following him, which is a bit unusual for family groups. It might be that given his age he was counted on a separate "single man"'s ticket and not part of the family, even though they travelled together.
http://trove.nla.gov.au/ndp/del/article/112791807?searchTerm=%22William%20Midwood%22"Among other missing beneficiaries are William Midwood, who left America for Melbourne in 1852"
This lead me to this notice in Lloyd's weekly newspaper in 1903:
http://newspaperarchive.com/uk/middlesex/london/lloyds-weekly-news/1903/11-29/page-19(in the matter of the will of Mary France, late of Honley in the parish of Almondbury, in York, spinster, deceased)
"William Midwood, alias James Greenalsh, a nephew of the above-named testatrix, Mary France, who left Providence, Rhode Island, in the United States of North America, for Melbourne, in the State of Victoria, in the Commonwealth of Australia, in or about the year 1852..."
(the rest of the wording is the standard if he is alive or if he is deceased but leaves a widow/children there is money for them).
The Greenalsh/Greenalch connection is interesting. There was an English born man of that name, a little older than William, around in RI (but had a son born there about 1845 according to the 1860 census so probably more likely someone they knew - or a relative?)
https://familysearch.org/ark:/61903/1:1:MFFX-3V4https://familysearch.org/ark:/61903/1:1:FS3W-Q3YI haven't been able to spot him in Australia yet - perhaps another name change?