Ultimately the TC/TCDS are the top level documents. They're also totally unusable by a normal pilot.
For that reason we have the POH. The POH is authorised by and listed in the TCDS. If there is any conflict between the two it's because somebody made a mistake - there shouldn't be. I was referring to the US TCDS because it was available, I would expect the UK TCDS to also agree with the content of the POH, or the UK version of the POH if that is any different (so far as I know it isn't).
Any aircraft has an approved build state. The aircraft, to fly legally, has to be in that state. So far as airworthiness regs are concerned, a U/S compass is similar to the wrong propeller type or flat tyres. It would be illegal from an airworthiness viewpoint to fly the aircraft in that state, even if from an operational viewpoint you're doing nothing wrong.
To use a simple analogy, if your car MoT has run out, it's illegal to drive it on the road - even if you obey all the speed limits and drive on the correct side of the road; it's driving an un-MOT'd car that's illegal, not how you were driving it.
Different types of-course have different equipment lists. For example, I think that an EGT gauge is MEL on Cessna singles? but not on Pipers. A manufacturer is likely to err on the side of a longer list of minimum instruments than required since it probably reduces the risk of stupid accidents for which they may get blamed.
G
N.B. Try Article 9 point 7. I'd say that anything listed on the TCDS or in the POH as "mandatory" is by definition "necessary for the airworthiness of the aircraft".
N.B.B. The more the merrier.