I am very sorry to hear about your losses. It is heart breaking to lose any pet - and I know how those ponies can cling to our hearts.
Thing is we still need to find out what's up so you don't lose your lassie too.
My best suggestion to you is to post in the emergency forum over at seahorse.org
other things you can do in the meantime....
You need to lower your tank temp to 72-73 it will slow down whatever is in the tank/horses.
I would also raise your SG SLOWLY to 1.025
I would continue with aggressive water changes - 20% of your total volume once a week.
find some test kits that give you numbers and not ranges... those all in one are ok for a quick reading. but they really don't help in solving issues in our delicate systems. ( I speak from experience

)