Have you been tested for POTS yet? The fact that salt pills, lots of water, Gatorade, and lying flat helps sounds like POTS. The confusing thing about POTS is chiari can cause POTS, or at least POTS like symptoms, but sometimes it's separate, so even if you have chiari surgery you may still be left with the POTS symptoms.
As for me, I had chiari surgery 5 weeks ago and I have seen a decrease in my symptoms and am very glad I had surgery. Before surgery, an increase in salt and lying flat would cause a worsening of symptoms for me.
You may have a little bit of everything going on, chiari, POTS and EDS.
For me, I now realize I had mild symptoms for a long time, but then something happened and it was like a light switch. I was functional one day, then bedridden the next. Then my symptoms would fluctuate from staying in bed all day to being out of bed but never feeling well. If I had to work, I would have had to quit. Just as Selma said, the change in seasons and temp affected me as well, also weather changes.
With the additional conditions you are DX'd with any one of them can account for your symptoms....Chiari symptoms cycle....so they can come and go and even flare up at times for unknown reasons....for me it was a change in seasons and temps.......
Some of us have gastrointestinal issues that can also cause some of the symptoms you mention......
Have you been tested for any GI, IBS or other issues in line like that ?