The reason I asked about any opinions whether or not William is buried there because I didn't know and didn't understand - here's why: It wasn't "quite clearly" written that William is buried there. I asked the question because I didn't understand about how things happen(ed) in Scotland. Here in the US, I have found (as one of many such examples) ancestors who are listed on the grave marker, but aren't buried there. Conversely, I have found records that show (or don't show) a burial in a cemetery, but the person isn't (or is) actually buried there. Further, Margaret is listed first with her maiden name and had died well before William who I have learned subsequently remarried. I asked about his burial, not whether he was "commemorated on the same stone" which he is; what's not clear is if he is actually buried there. Sorry for not understanding and thanks for the help.
Apologies for any misunderstanding.
First, it is normal for a married woman to be referred to by her own maiden surname in legal documents and on gravestones and census etc, because in Scotland a married woman does not legally lose her maiden surname on marriage. This is why you generally get the mother's surname in baptism records.
As to whether or not William is actually in the grave, my choice of words was quite deliberate. The overwhelming probability is that he is buried there, but there is room for a sliver of uncertainty.
Records of burials are fairly scarce in Scotland before about the 1840s, and even later in many places. I do not know if the burial records for Walston have survived, but if they have they will either be in the parish register and therefore in the church records on Scotland's People, or, just possibly, there might be mortcloth records in the relevant kirk session minutes, which are supposed to be becoming available 'soon' on Scotland's People.
However the likelihood is that the gravestone inscription is the only record that is available of William's death.
The book of pre-1855 MIs uses a very abbreviated format.