If Mahamrityunjay mantra was described in Vedas very before then why it was said that it was composed by rishi markandeya? Does it was added later in the puranas to highlight this mantra?
Rig Veda VII.59.12 mentions this mantra.
तर्यम्बकं यजामहे सुगन्धिं पुष्टिवर्धनम |
उर्वारुकमिवबन्धनान मर्त्योर्मुक्षीय माम्र्तात ||
Tryambaka we worship, sweet augmenter of prosperity. As from its stem the cucumber, so may I be released from death, not reft of immortality.
According to Rig Veda Anukramani, most of the mantras from Mandala VII, in which Mahamrityunjay mantra finds place, and especially this mantra, were composed by VasiSTha MaitrAvaruNI.
So sage Markandeya was not the composer of this mantra.
Now the 2nd question - Does it was added later in the puranas to highlight this mantra?
It is very difficult to say as to why the authorship of this mantra was attributed to Sage Markandeya, as major part of Puranas was written, deviating from Veda.