Do instrument packs add content ?

They actually add different models to Pianoteq. And no, you cannot tweak D4 to a marimba - models are essentially different. So, a marimba model doesn't add anything to other piano models, like D4, K2 or Blüthner - it stands on its own.

Modartt produces its own sample library of a given piano, then uses that to 'profile' the instrument's basic properties you described (strings, cabinet, frame, soundboard impedance).
I'm not sure if the strings are defined in terms of real physical parameters or simply precalculated oscillators that reflect the "heart" of the instrument. What I can say for the pianos,
and some other instruments like the harp, harpsichords, and others:

Each string has a longitudinal mode that's set to a certain pitch at the instrument's real string length. On smaller pianos it is typically sharper by 1 to 2 semitones in the bass bridge compared to a concert grand, but the string lengths are much shorter and thicker. Thus, it's possible to achieve a much lower-pitched longitudinal mode than a real piano could ever produce, since the model is based off of the scale design of the original instrument, which does not change as one changes the string length. Even adding only one meter to the D4's case length (string length) puts B-1's longitudinal mode a semitone LOWER than the Bosendorfer Imperial 290's low B! The Klavins 370, having thinner and longer strings, keeps relatively typical pitches for its longitudinal modes, which lie around either dominant 27ths or perfect 29ths, give or take a few cents.

The bass bridge most prominently highlights these changes, but the plain wire section is more subtle, perhaps more annoying. Try maxing the string length on the CP-80. Notice how the middle sounds a little sour? That's the L-mode rearing its ugly head.

Here's how a long piano sounds like. The example is the Alexander Stadium Grand, case length 570cm, all plain wire.

Oh, and all that above does not include the precomputed hammer samples for each instrument!