I rarely consult the Thunderbird address book for anything other than a phone number. It is unnecessary for email addresses as these auto-complete in message correspondent fields, and how often does one use a physical address now-a-days? I generally look up phone numbers via search in the address book window and have the major phone number types (work, home, mobile) displayed as columns. So I almost never open an actual card.
Is this usage unusual? I suspect not.
I've got many hundreds of address book records (recently culled from thousands) spanning decades of contacts of all types (business, personal, ...). If the address book columns are simply first, second, ..., that would be unusable. Who can remember whether the first is a home, work, mobile or fax, especially when contacts created in gmail lead to a random order in thunderbird? The address book window would become useless, creating a requirement to open every card before using a number, which would decrease the productivity of address book substantially.
To me getting phone number types right is more important than anything else. I would suggest that a person syncing Thunderbird and gmail can only effectively use phone number types that both support. gcontactsync should work well for that case. I think all you need to do is to ensure you place the built-in Thunderbird types (Work, Home, Work Fax, Mobile, Pager in TB 2) into the correct positions (First, Second, Third, Fourth, Fifth, respectively). I don't see why that would affect any translations.
I did not previously use another sync extension, instead doing it with import-export, usually exporting from TB, replacing the header line with one that uses Outlook field names, and then importing to gmail. Your extension of course makes this much easier, when it works.
School and job and these extensions? Good for you! Have you open-sourced the extensions? (Sorry if this is dumb question since I didn't look to find out.) Perhaps other users could submit changes to fix this problem, maybe even me.