Hi!
I'm really sorry about your frog. You're in good hands with Essjay helping you, she knows her stuff.
Just wanted to explain why this sometimes happens, because you're not the first person to be like "what the heck? They seemed fine when the levels were high, I'm improving it now, and that kills them??" and it can cause doubt and confusion. But there's a good reason why it happens. What you have to know is that ammonia and nitrite are so toxic, they literally burn the skin and gills. Like breathing in smoke, which we can handle in low levels for a short time too, but enough smoke in the room you're trapped in and it proves fatal.
The ammonia and nitrite irritate and inflames the tissues in the gills and on the skin, irritating the animal and making it hard to breathe. Levels get high enough and the animal passes away from the damage caused.
Even low levels cause damage, kill more sensitive creatures, and especially if ongoing over time can cause enough damage and stress to kill the fish/frog/invert. Once the water is safe again, the animals aren't instantly recovered, they still have damage from the ammonia and nitrite, and it takes time for them to recover even once in clean water, and not all will survive, sadly.
I often use fire and smoke as an analogy for this. If someone survives a housefire with smoke inhalation and some burns, if they later pass away in the hospital, it wasn't being moved to the hospital and the doctors treating them that killed them! They just couldn't recover from the damage they'd taken as a result of the fire and smoke. Same goes here for the creatures in our tanks.
So, keep listening to Essjay and the others, they're right on the money, and you do seem to get the cycle. It's going to take patience and a lot of water changes to get it fully cycled, but the daily water changes aren't forever! It can be rough getting a tank cycled, especially when accidentally overstocked, but it will stabilise eventually.