Why you can't reverse cheese curdling in fondue?
Most proteins undergo irreversible structural changes when heated. The exact temperature where this happens depends on the protein. In most cases, no covalent bonds are broken or formed during this 'denaturation' process.
But the irreversible nature of the changes means that there's no good way to rescue a curdled fondue sauce (or a curdled egg custard).

Other components present in the environment of the protein (like alcohol, salt) can influence the denaturation process.

In your case, when you first get the wine to simmer, your temperature might just be too high from the start to prevent denaturation of the cheese proteins, suggested temperatures for melting cheese are around 150F (~66°C). And of course, different cheeses have had different treatments during production (some are already heated to 50-55°C), and can have different behaviour when heated. And duration plays a role (the denaturation isn't instantanuous).