it's the country changes that mess it up - countries changing their names and/or merging, splitting in two are a bit of a headache.
I don't think you need to worry about that. Where a name changes, just include the data for where the country was under that name and leave the other years blank. It's probably much simpler that way. We will be able to piece together the rest.
A classic example will be Burma which became Myanmar, possibly switching names back more than once. You could just place the two sets of data next to each other in the list. No need to merge the data.