Hello Colin,

I have Fife ancestors, and had a couple of credits left on ScotlandsPeople, but came up with no marriage anywhere in Scotland for your couple in 1868.
I then tried 1860-1874, and there was one only entry for their names. I checked each year (no match) until I ended up with one entry in 1860. Could a "0" written somewhere have been mistaken for an "8"? If her age given in the 1911 census was correct she was old enough to marry in 1860.

I then restricted the search to exact surnames, exact forenames - still the one entry. I added Fife - no match.
I then tried nearby places with ports, and got the hit with Aberdeen. (This was all free.)

I then used 1 credit to look at the result, which showed:
"You searched for: Male Surname: "mcdonald"; Male Surname Option: Exact; Male Forename: "john"; Male Forename Option: Exact; Female Surname: "wood"; Female Surname Option: Exact; Female Forename: "elizabeth"; Female Forename Option: Exact; Year From: 1860; Year To: 1860; County: "ABERDEEN"
Year/Surname(M)/Forename(M)/Surname(F)/Forename(F)/District/City/County/MR/GROS Data/Image View (5 credits).
1860 MCDONALD JOHN/ WOOD ELIZABETH/ ST NICHOLAS ABERDEEN CITY/ABERDEEN/168/01 0068

I think it would be worth looking at this entry, because if it turns out to be the correct one it should answer many of your questions.

Scottish marriage certs contain:
Registers of Marriages Date Range
Bridegroom: name & marital status 1855 to date
Bridegroom: age 1855 - 1971
Bridegroom: date & place of birth 1855, then 1972 to date
Bridegroom: rank, profession or occupation 1855 to date
Bridegroom: residence 1855 to date
Bride: name & marital status 1855 to date
Bride: age 1855 - 1971
Bride: date & place of birth 1855 only, then 1972 to date
Bride: rank, profession or occupation 1855 to date
Bride: residence 1855 to date
Couple: date & place of marriage 1855 to date
Couple: if irregular marriage, date of conviction etc. 1855 - 1965
Couple: relationship, if related to each other 1855 - 1860
Parents of couple: name including maiden surnames of mothers 1855 to date
Parents of couple: rank, profession or occupation 1855 to date
Parents of couple: alive or deceased? 1855 to date
Children of couple by each former marriage 1855 only
Witnesses: signatures 1855 to date
Witnesses: addresses 1922 to date
Marriage celebrant: signature 1855 to date
Registrar: date & place of registration, signature 1855 to date
Form of Marriage e.g. Established Church of Scotland, Free Kirk, Licence (type) etc 1855 to date.

Please note that 1855 differed from later years.

You will need to register and to buy a minimum number of credits on ScotlandsPeople by credit card, but they are not expensive (30 credits for £7.00). You can view the entry online immediately, and they keep your paid results on the website so that you can return to them at a later date.

Please let us know what you find!

Sue