Explanation:

For the first question, only ExpressRoute GW SKU Ultra Performance support FastPath feature.
For the second question, vnet1 will connect to ExpressRoute gw, once Vnet1 peers with Vnet2, the traffic from on-premise network will bypass GW and Vnet1, directly goes to Vnet2, while this feature is under public preview.
====Reference
ExpressRoute virtual network gateway is designed to exchange network routes and route network traffic.
FastPath is designed to improve the data path performance between your on-premises network and your virtual network. When enabled, FastPath sends network traffic directly to virtual machines in the virtual network, bypassing the gateway.
To configure FastPath, the virtual network gateway must be either:
Ultra Performance
ErGw3AZ
VNet Peering - FastPath will send traffic directly to any VM deployed in a virtual network peered to the one connected to ExpressRoute, bypassing the ExpressRoute virtual network gateway.
https://docs.microsoft.com/en-us/azure/expressroute/about-fastpath
Gateway SKU
https://docs.microsoft.com/en-us/azure/expressroute/expressroute-about-virtual-network-gateways