1. Opening switches, fuses and breakers can protect other devices like transformers that may be harder to replace than wire.
Exactly. In the one confirmed case of an EMP managing to do damage to a coil winding, it involved a Nuclear EMP in Russia during the 1960's, which melted to coils of the diesel generator that was providing power. It just so happened to also be connected to 22 miles of cable.
So plenty of wire able to induce both a current and a voltage within that closed system by way of induction. The coils in the generator evidently proved to be the "weakest link" for finding a path to ground, so it was what got slagged.
It also didn't help the Russians that the generator was plugged in and active at the time the EMP hit.
2. From what I've read, the length of the wire impacts how much charge builds up. so segmenting lines into shorter sections by opening the switches will reduce the charge buildup in any one section.
It gets a bit more complicated than that, but the basic answer is this. The shorter an "individual wire run" is, the less voltage the line in question will be able to "absorb" via induction.
The "issue" with circuit breakers alone is the matter of the "spark gap" that is left after the switch is "thrown" and a question of is the line in question will generate both enough voltage and current that it manages to
1) "Jump the gap" to the next link in the chain.
2) Do damage when it does "make the jump"
Which is I why I opened by making comments about either physically disconnecting and/or physically grounding out specific power lines.
If there is a large physical disconnect, above and beyond "just a circuit breaker" in the mix, that provides an extra layer of protection, as the "spark gap" just became significantly larger.
Likewise, with a solar EMP, you're dealing with a wave, not a pulse, as would be seen with a nuke. As such, grounding out a power line for example may be more than sufficient to prevent it from being placed under a induced load capable of doing damage to the power line itself.
3. Most of the primary (12 or 4 kV) overhead conductor in the US is bare wire. The insulated wire you see is either secondary (220 or 110 volts, what comes into your house) or is cable(tv) or telephone wire. I'm not sure if a solar EMP will affect underground conductor at all.
Underground cable wouldn't be able to "induct" much, but it would still be subject to some within its own system as it does come up to the surface level periodically over the course of its "run" as it enters into/exits various substations or interconnects. By and large, "throwing the circuit breaker" would likely be up to the task of protecting them from a solar event... So long as it was isolated from the above ground portion of the power grid. (This would also include the customers themselves, as they too could "backfeed" onto the grid if they're not careful. A typical new construction building of any size, be it commercial or residential, is likely to potentially have miles of electrical cable within it. So plenty of pathways for inducing voltages in an EMP scenario. Thankfully for the power company, not all of that cabling will be theirs, some will be telephone services, others will be television services, and then there are the network services. But so long as everybody on a given "circuit" turns has their stuff shut off at the breaker, things should be well enough. The issue at present is that while many power companies can remotely throw breakers on their own equipment, they can't do this on the end-user side. I wouldn't be certain as to their ability to remotely shut things off "at street level" (before it gets to the customer) at this point in time. Maybe in newer constructions, probably not so much in older areas.
Obviously fiber-optics are immune to EMP in a direct sense, but as fiber optics require transceivers which require electricity in order to operate, and you need to have a relay every so often....
As to most "insulated" wiring across the world? It isn't THAT insulated. Those wires are minimally insulated in accordance to the "normal operating specifications" of the device/install(for which an EMP event is obviously not considered "normal"). A Solar flare would be able to induce voltage across much/most of said wiring, but in particular so long as said wiring isn't energized at the time of the EMP event(and it was "electrically isolated" when it happens, so that "external wiring" doesn't help increase the amounts of voltage/current it gets exposed to), it's probably going to be fine.
If you were wanting wiring that stands a chance against an EMP while in use, you're talking about "shielded" cabling, which is basically a cable which is then wrapped in an additional electrical insulator, which is then once again wrapped in yet another layer of conductive material which would then be tied to ground. As such, most "shielded cable" is often also referred to as "armored cabling" but not all "armored cabling" is actually shielded, as it hasn't been tied to ground, among other considerations.