The first two bars are a G major chord. The next two are a D7 chord. Nothing ambiguous about that (although, of course, a re-harmonisation is always possible).
If your task was to orchestrate, do just that. Keep it as a unison texture. Your job is to choose what instruments to use, not to re-compose it.
Mozart filled in the G chord at the beginning. ...
What's wrong with orchestrating it exactly as Mozart wrote it - a unison C?
You are absolutely right, it is harmonically ambiguous. That's the whole point of it. Making it unambiguous is as stupid as writing a three-volume treatise explaining why the chicken crossed the road, IMO.
Mozart clearly implies a D7 chord, but if you want to write a C major chord there's nothing to stop you (but maybe you'll hear Mozart turning in his grave). If you do decide to write D7 than you have to make it work by voicing it well. That almost certainly means starting with the root in the bass instruments (including cello/bass). But if a unison C was ...
Articulation signs are signs indicating what kind of articulation the composer wants. He might not think about which technique the performer will apply.
Martele is a technique that bowed string players can apply. If your sheet music has indications of which bow technique to use it is often the publisher's suggestion.
I am both a string player and a ...
Some more examples of the piano in the orchestra, courtesy of Walter Piston, Orchestration, p.341--6:
Stravinsky: Petrushka (1911)
Bartók: Dance Suite (1923)
Aaron Copland: Symphony 1 (re-orchestrated without organ, 1928)
Prokofiev: Symphony 5 (1944)
Bohuslav Martinů: Symphony 4 (1945)
Aaron Copland: Symphony 3 (1946)