Yes, it's worth moving from cable to fiber. I'd pay a premium to do so, but it's often cheaper.
You didn't specify your fiber provider, but in most cases, they will provide you with an ONT during installation, which is something that converts a fiber connection into an Ethernet connection. They also usually provide some sort of gateway or router too, but that can either be optional or required from them, depending on the ISP. You just need to connect this ONT directly to your existing router, or put their required gateway into "Bridge Mode" or "Passthrough Mode" first and connect that to your existing router.
Either way you usually get a choice to where they put the ONT / gateway. In your case, you can have them install this right where your modem (and router) is at today, right next to each other. As long as that area is accessible, they can probably use the same hole as your coax coming from the outside into your basement.
It's very likely the install will mean no changes for your network at all, other than retiring the coax and modem, and now using their ONT. If they require use of their gateway, then just put it into Bridge Mode as already stated. Just think of the ONT as your new modem for fiber.
Just be present during install, and show them where you have things right now, and the installer will work with you to put it in a good spot.
Scenario 2 is more accurate for BOTH cases, but not because you used a switch. Scenario 1 is false.
It's the ISP provided speed that is the limited resource. You have 100 Mbps of Internet service, which can be consumed by any single connection. If multiple connections want to use Internet, that speed gets split up between all the devices trying to use it simultaneously.
It does not have to be in equal pieces. If one connection only wants 20 Mbps of Internet, another connection can use the other 80 Mbps remaining.
Using a switch does not affect the consumption of Internet from devices unless the switch port speed itself is lower than your maximum ISP speed. This isn't true in your example.