I think the first thing to do is make sure the upstairs unit is operating properly in all aspects including the sizing, ducts, pressures, return air, dirty coil? etc

New carrier coil, balance/heat load are good. They said I might want to put a air intake in the big room upstairs though.

so you second story is smaller than the first story, based
on size of systems.
my advice is not hvac realated but house problems.
being in the same type climate as you (louisiana)
we have this complaint often.
think about this..
this second story is surrounded by hot attic air.
between the ceiling and roof, between floors and on the
sides where the interior walls are shared by attic.
if it is 100 degrees outside, then your attic is 130 to 140 degrees.
(see southface institute air sealing factsheets,
building science hot humid climate builder's guides)
while there isn't much you can do at the roof/ceiling
that is economically feisable, you can stop the heat gain
through the walls, and the air leakage. if you look at
these attic walls from inside the attic you'll prolly see
insulation bats..(hopefully well fitted into stud bays!)
while this helps, the air movement through this insulation
reduces the efficiency of the insulation.
the key is to stop the air movement and reflect the heat back into the attic.
the best way to achieve this is to use foam/foil faced sheathing boards. they are easy to work with, cut with a razor knife and attach with buttoncap nails. install the sheathing board with foil facing into attic space, in a continous manner to the bottom of the wall, nail well and seal seams in foil board with foil faced tape. this will
stop air movement and the foil will act as a radaint barrier and reflect heat out of walls. Caulking at top of
wall and bottom to stop air movement. while builders
seal the sole plates on first floor walls, they seldom do the same on second floor, this will effectively seal these areas from the attic.
to stop the air movement between floors takes more time and effort. the same foil board should be cut to fit between ceiling joists of first floor, caulked to seal
to joists and connected to foil boards installed on walls.
if you look at the fact sheets on southface's site it will
show you in pictures what I am trying to exlplain in words.

another option which is easier, but much more expensive is open cell foam (again climate specific type of foam) installed on roof rafters to make an unvented attic..building science site has quite a bit of unvented attic info. however...to make a good seal the foam must be installed correctly.

I have personally done the types of install with the foil board and reduced the air infiltration to second story
to acceptable rates. Testing homes with a blower door has shown this to be common leakage areas that have a huge effect on the comfort of these upstairs and bonus rooms that are surrounded by attic temps.

your options 1, 2 and 3
solar vent fan...no no no..these create negative pressures
making house leakage worse, and duct leakage worse.
radaint barrier..will help, but the wall area will still
be leaky and radaint barrier will not lower attic temps
enough to achieve level of comfort in second story.
portable a/c & dehumidifier...no to the a/c but possibly
for the dehumdifier..I like dehumidifiers.
however they won't be of much benefit when only used for short periods of time. it takes time to lower RH and to maintain RH below 50%, dehumidifier use will be constant.

best of luck

http://www.southface.org/web/resources&services/publications/factsheets/sf_factsheet-menu.htm

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