When you create a VPN connection with Amazon VPC, Amazon offers a configuration file for different router brands. This file can be downloaded as soon as the VPN creation is done.
So the natural procedure is to start the configuration process in Amazon and then just run the provided commands in your router.
My question is, can you do it the other way around? Is there a way to modify the VPN settings in Amazon based on my physical router settings? Or are Amazon VPN settings just read-only?
Yes, you can modify various VPN settings in Amazon.
For an existing VPN connection, you can edit the static routes for your connection from the VPC Console (in the navigation pane, choose VPN Connections) - see Editing Static Routes for a VPN Connection.
You can also specify many options when manually setting up a new VPN connection - see Setting up the VPN Connection in the Amazon VPC documentation.
For more general info on Amazon VPC/VPN connectivity options, see the Amazon Virtual Private Cloud Connectivity Options whitepaper.