VLAN hopping is only possible due to misconfiguration. I'd like to be proven otherwise if that's not the case. VLANs are used EVERYWHERE where it matters. And no, the single port is absolutely not a bottleneck because the port is full-duplex.
If you're trying to push close to a gigabit up and down simultaneously that single port will become a bottleneck. I agree for most typical use cases it is not a concern.
Both get 500Mbit.
Bottleneck.
As someone who works a programming job from home, I can tell you that the cases are not at all contrived.
I've also heard from folks who have lots of roommates (whether or not those roommates are their swarm of children) that heavy simultaneous upload and download traffic is very common.
You can get away with a router that has a single physical port. It's generally just easier and better to have more than one physical port... and even 10gbit-capable ethernet ports are pretty damn cheap. (I can get two 10gbit ports for about the same price as two 1gbit ports... ~30 USD.)