Don't fret about spellings, they weren't fixed until fairly recently. Many people couldn't read or write, so the minister would simply write down what her heard. Add to the mix that in rural parts of Kintyre, Gaelic was spoken, and whoever was writing the info had to transliterate. The old registers weren't filled in when the baptism or marriage took place, but written up later from notes. Even these days, that can cause problems!
Achnacarnan, as it is spelled today, is a farmhouse located to the east of the main road not far north of the Kennacraig ferry terminal. I can't recall if there is a sign at the road end. Most of the farms have a wee sign, but we not talking huge signposts here! It is on modern maps and was also on the first edition OS map from the 1860s. If your people were farmers, then they would have lived in the farmhouse. If they worked as ag labs, then they would have lived in small, single storey, two-room dwellings on the farm. Sometimes the term 'farm town/ferm toon' is used, but it wasn't a town - simply a scattered handful of buildings.
I think one of the ruinous crofts on Achnacarnan land has been restored to make it habitable. When you are in Kintyre, you will see there are numerous examples of roofless, abandoned crofts. Quite a lot are used as sheep fanks. As a wee kid, I remember the sheep being rounded up and kept in one of these ruined crofts as the sheep waited to be sheared. The building was where my ancestors had lived in the early 19th century.
You ask about cemeteries. It depends what period you are looking at. However, as a rule of thumb, people would be buried in their birth parish. For people born in Achnacarnan, it would probably be Gartnagrenach near Whitehouse. Check out
www.ralstongenealogy.com which has an excellent section on Kintyre cemeteries. I had a quick look and there are a few McKinvens in Gartnagrenach - a couple are quite early: Alexander McKinven who died in 1809 aged 69, and 85-year-old Gilbert buried in 1838. But, of course, many families never erected headstones. Tarbert cemetery is also a possibility.
Finally, in May, there is a Kintyre Way Relay (the Kintyre Way is a king-distance walk). The Ultra version is being organised by the current occupant of Achnacarnan.
Enjoy your visit - it is a beautiful area.