Box 1: Yes -
Resources in Subnet1 will use the Route2 and its Next hop ID address to the Firewall to reach the Internet.
Box 2: Yes -
Yes, with network network peering.
Box 3: No -
Resources in Subnet2 can only reach resources in Subnet1, as gateway transit for virtual network peering has not been configured.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-udr-overview
https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-peering-gateway-transit