So the claim reportedly made by the stone restoration company that they did not seal it because the stone is already naturally acid-resistant (even if it were not, sealing would not help one bit), doesn't make much sense.
That duly said, the problem the way you're reporting it puzzels me quite a bit. When I read your first posting my mind immediately raced toward the conclusion that, perhaps, they had "crystallized" your stone. But then, when you so specifically mentioned that they used 5 X yellow powder, the riddle became, well... a riddle.
On the other hand, how could they possibly polish serpentinite with 5 X? If your green marble is serpentinite (and that is what I obviously assumed by the fact that in 5 years you did not have any etching), 5 X should have not even tickled it. If it did, it must be ophicalcite instead. But then , ophicalcite would have etched in 5 years and never passed your pre-purchase test!... Hmmm...
The only conclusion that I feel like venturing is that, while your stone is in fact serpentinite, it did react chemically, somehow (and strangely) with the potassium oxalate of the 5 X powder and created a Beilby layer that's sensitive to acidic substances.
It doesn't make sense, but... it's the only possible explanation that make sense!...
What to do?
Have the surface of your stone professionally re-surfaced and polished with either a "cold" polishing powder for granite (5 X is a "hot" polishing powder), or with specific dry diamond pads. In that way, there could not be any chemical interaction, the polishing would be strictly mechanical and the Beilby layer on the surface of your stone would strictly be the stone itself. I do know that in the factory they never use "hot" polishing elements to polish green marble.