now here's something interesting - death report in the Merthyr papers from 27 August 1864, for a James Harrison Youd, in his 54th year, of Bridge St, a silk dyer. It says on Tuesday last, which would have been 23 August, two days later than the date given for William in the probate registry.
I can also see reference to the Bridge St Dye House, next to the Carriers Arms, run by an E H Youd , late of Liverpool, in the 1850s. So there are other Youds about, even if not showing up in the census, who may have a William closer in age to Eliza than the one in Bridge St