I have the INTERIOR dimensions of the walls, doors, etc, and figured I could just estimate the exteriors, but presumably due to human error everything is ending up just slightly off (ie, "well, I know this wall is 83cm, but it doesn't line up with this other wall unless I say it's 80.25cm") when I do it this way.

The misalignment is probably because you have the wrong wall thickness. And there are probably different wall thicknesses between the different walls. That will take some calculating.
For interior decorating it is only important that each room matches the inside dimensions. Then you can change wall thicknesses if rooms don't align since changing the wall thickness decreases the inside and you have to move a wall on one side to get back to the correct inside measurement. Try to figure out what the wall thickness is of the interior non-load baring walls. These are usually all the same. Probably something like 10-15cm depending if it's poured concrete or stones with plaster.
Don't be surprised if some things still don't line up, a house is never perfectly aligned. Sometimes there are differences of cm's instead of mm's between wall lengths on opposite sides of a room. Perhaps you already found some when taking the measurements.

I guess I will need an extra set of hands to help me get an accurate measurement of the exterior walls and then fill everything in from there, based on the guide you linked - is that better than doing it my way, of taking all the "interior" measurements and then extrapolating the exterior dimensions, or does it not really matter?

The exterior measurements are of little use, you are going to redecorate the interior so the important thing is to get the interior dimensions correct. Maybe just part of the exterior where the balcony is.