Hi
Looked at GFHS MI fiches at Caernarfon Archives this afternoon. Didn't have much time; didn't spot anything referring to Cellar/Sellar at St Beuno, Botwnnog or St Pedr, Mellteyrn. However there are 3 graves (out of 10) in Baptist cemetery Tyddyn D---- (can't understand my own writing, will check !). Inscriptions in Welsh - basic info below:
002 Robert Jones, son of John/Ann Jones, Cellar, Botwnnog, 11/11/1925, aged 53. Faithful deacon with the Baptists at Portmadoc for 13 years. Also Jane Jones, 02/12/1938, aged 74.
003 John Jones, Cellar, 24/06/1886, aged 76. Also Ann Jones his wife, 01/02/1901 aged 72.
009 Catherine wife of Griffith Jones, Cellar 23/01/1939, aged 68. Also Griffith Jones, 26/01/1951, aged 88.
There are a number of other churches not far away. It's possible that burials have taken place in other locations connected with the family, or nonconformist cemeteries.
Are the names above amongst those you're interested in ?
Emyr