How to say whether INL and DNL of a DAC is good enough?
What you have to realize is that the DNL measure is a noise-like measure.  Meaning you can use it in noise calculations, simply put it is a RMS measure.  That means that it's entirely possible to have low DNL and still not be monotonic.  Monotonicity can only be guaranteed architecturally.

Your argument for a reduced resolution implying that it then must be mono-tonic is valid in some cases.  As an example if you take the MSB of any DAC and toggle it, every DAC is monotonic. Moving from 14 bit to 13 bit isn't necessarily clear.  It depends upon architecture, a Sigma-delta DAC probably would be.

Some DACs are made of cascades of different techniques so it's hard to say without the datasheet.

INL being "good enough" depends upon you application.