You would have to get a GC out to inspect the attic and then give you a written recommendation of alterations, which you then would take to your association. Talk with your neighbors and see if they have the same issue, them jumping in on the issue would offer a greater chance of approval. It is possible the association may pay for it if the problem is attic venting, insulation would likely fall on you.
Forcing the system to run more to compensate for the upstairs would drive up your electric bill and may not actually help if the problem is in the attic or balance of the HVAC. You could set the temp lower at night to accomplish the same thing as dual sensors, but you still are just throwing money away that should be better spent on addressing the issue directly.
Having a nest vs non-smart thermostat allows you to set schedules in a smarter way than a regular tstat.
Having dampers allows you to balance the system so the downstairs gets less air than the upstairs, instead of you having to go around closing vents and the downstairs would be cooled better than just shutting vents and sending most of the air upstairs.