If I understand you correctly there are really two questions in here:
1) Can Tableau cope with different forms of postcode/zipcode?
2) Can Tableau fall back to less and less precise geocoding, based on different geocoded fields, until it finds a match.
The answer to 1) is yes and I'm fairly sure the answer to 2) is no (or at least I can't think how to achieve that).
For the first case, all you need to do is have country and postcode fields in your data (with their geographic roles set appropriately) and place both on the view. The postcode field just needs to be a text field with the relevant format for each country. Tableau sorts out making the right association based on the Country.
Thanks for the answer, Do I understand correctly your suggestion that canadian postal code and US zip code could/should be in 2 different fields. If so can I have multiple fields that are refered as "postal code"?
On the other hand can I assign a single geographical role to a field and have 2 fields called longitude and latitude that Tableau can use to mapped different address? Our students come from pretty much anywhere in the world and I would like to map their origin.
No, just a single [Postcode] (or [ZipCode]) field which has the postcode in whatever format applies to that country as well as a Country field which has the Country in one of the formates that Tableau understands (which can be the full name (including quite a few variants) or it can be one of various abbreviation formats.
You just need to make sure that both [Country] and [PostCode] have their geographic roles set appropriately and you need both to be on the view.
Another question on this. Is there any way to "force" tableau to use the custom geocoded file (one that has longitude and latitude)? Is there any way to know which geocode table tableau is using? Finally, I've read somewhere that the column name are case sensitive (latitude vs Latitude), and has a set of rules for their. However, I could not find information on this.
You need to make two different files, one is an .ini and the other is a .csv
I don't think it really matters what you call the columns as long as you make sure they match in both files, you will have to change the data type to a number and tell Tableau it is a Long or Lat for the Geographic Role.
Here is an example of one I made yesterday that works:
In a file named "My Custom GeoCode.ini" I have the following:
[My Custom GeoCode.csv]
In a file named "My Custom GeoCode.csv" I have the following that looks like this fake data: