flyingfalcon16,
Here are your answers:
... applying nose down trim, will produce the effect of adding negative pitch attitude.
Not necessarily. The nose down trim will create the
tendency for the nose to pitch down. This "tendency" is better known as a "moment". There are many pitching moments acting at any given time. Which way the aircraft pitches, if at all, depends on what the sum of those moments is. If the sum was zero, for example, and nose down trim was applied, then yes, you would expect a pitch down result, but the application of nose-down trim does not guarantee a nose-down, or negative, pitch attitude.
Note that a pitch-down does not necessarily result in a "negative pitch attitude". In aerodynamics, the latter is considered to be a pitch attitude whose angle is below the horizon. An aircraft with a pitch attitude of positive 45 degrees, for example, could experience a pitch-down such that it's pitch attitude was now positive 30 degrees. Both these attitudes are considered positive pitch attitudes.
So MCAS when activated is literally pushing the nose down. MCAS is sending nose down trim to bring the nose downward.
MCAS creates a nose-down pitching moment. As stated above, that moment may or may not result in an actual pitch-down.
Does trim effect pitch attitude directly or is it providing column force so the pilot changes pitch?
Trim changes create changes to the pitching moment. This is true for everything on the aircraft. Extending undercarriage and flaps, increasing or decreasing thrust, etc - all create pitching moments. At any time in flight, there are many pitching moments. Whether the aircraft pitches up or down, and at what rate, depends on the sum of all those moments. If the sum is zero, for example, then there will be no pitching.
Would MCAS ever not be trying to bring the nose down?
MCAS is there to create pitch-down moments only, and only when the aircraft is at high angles of attack (AoA) - supposedly.
Is it inaccurate to say MCAS uses nose down trim commands to apply negative pitch attitude to the plane?
Yes, this is inaccurate. The accurate statement would be, "MCAS uses nose down trim commands to effect pitch-down moments". This is the pure intent of the MCAS - the pitch-down moment - nothing to do with the attitude of the aircraft.
... does adding nose down trim add negative pitch attitude?
Not necessarily. See above.
Would MCAS ever be active adding trim in a scenario of high AoA and the plane has negative pitch attitude?
As I understand it, yes, the MCAS could apply trim in that scenario.
Is MCAS literally part of the trim system?
No. It is a completely separate system, whose only means to effect a pitch-down moment is via the stabiliser trim. Boeing would have dearly loved to have had other ways available to do this, but did not.