It's sort of sad that "cha" and "tha" sound so similar but have such different meanings.
The vocabulary says this also translates as the Sabbath. Why would I be marked wrong?
Possibly just because the creators of the course didn't add that as a valid answer. But it could also be that perhaps the Sabbath is considered ambiguous - for Jews it would be Saturday, not Sunday.
I do not work on Sundays should be accepted, as the future tense can also be the present habitual?
I tried I do not work on Sunday when it came around again and it was accepted. I think the plural is more common in the British English present habitual when the day isn't qualified, e.g. I work every Monday vs I work on Mondays.
Can "na" mean "or" OR "on"? Or do I have a typo in my notes?
You have a typo. Or in Gaelic is no.
in the introductory notes Sunday was given both as latha na sabaid and la na sabaid (with accents on the 3rd a, of course)... ???
Yes, là is an acceptable alternative to latha.