Why is it "taflen waith" but "taflenni gwaith"? Why does gwaith mutate in the first place (is it an adjective and not a noun?)? And why doesn't it after the plural?
Here it is acting as a adjective. Adjectives mutate after feminine nouns in the singular but not in the plural (adjectives never mutate after masculine nouns no matter if they're singular or plural.) So since Taflen it feminine it makes the phrase Taflen waith, but in the plural it is Taflenni gwaith because of the afore mentioned rule.
Would "taflen waith" also be used to describe a worksheet within Excel?
(other spreadsheet software is also available)
What is a work sheet? It's not anything taught in the Swedish schools (but then again, we're not taught much at all in school).
It is a sheet of paper with some work to be done on it -- especially at school, where the teacher may hand our worksheets with maths problems or English practice (e.g. "form the plural of these nouns:") etc.
Try a Google Image Search for the keywords "worksheet" or "Arbeitsblatt" and you will get a number of samples of different worksheets, many of them for schoolchildren.