There's an item in a couple of Welsh language newspapers, 1907, about the visit to Llansamlet of two former residents "Mr Hopkin & Mrs Gweni Sims, Troy, Illinois" - parents "Tomos and Mari" (Sims) and "Twmi Dafydd and Beti" (Thomas). I've been checking back from there and revisiting the Llansamlet records.
Michael,
A warm welcome to Rootschat.
Re the "John, born 1858, son of David and Mary .... the one who immigrated to the US" -
There's a baptism (transcription) for a John Sims, 3 Aug 1858, Llansamlet, parents David and Mary, abode "Pentre Dwr". There's a marriage for David Sims, Jun Qtr, Neath, 11a 683 - on the same page Mary Jenkins and Mary Beddows.
A 3yr old John + brother David 7 mths, with parents David 29 and Mary 24 can be found on the 1861 Census - address "Old Halfway". Either side of this address is "Pentre Dwr" (the Halfway House pub was in Pentre Dwr). I think this John can be tracked through to the 1911 Census - he didn't marry.