Is the measure word optional in 这件毛衣?
In Mandarin, especially Northern China dialects, measure words can be omitted when

It is following a demonstrative pronoun (e.g. 这, 那), AND
The numeral is 1 (one), AND
It is not a collective measure word （e.g. 些, 群).

The grammatical role of the phrase does not change. E.g.

这一件毛衣 = 这件毛衣 = 这毛衣
这一些毛衣 = 这些毛衣 collective measure word 些 cannot be omitted.