Swamy,
Good questions. I know because these were problems I had when I first started using AWS and Scalr.
ROLE CREATION AND SUBNETS
I created the VPC and 8 subnets (4 public, 4 private) using Terraform. My next step was to go into "Governance" in the environment and work with the "AWS | VPC". There you can set the region(s) and then select the "Allowed VPCs". So, after selecting a VPC you can configure "". In my case, I gave my users access to 1 public and 1 private subnet. I did this so they did not ask me questions about which subnet is best.
Finally, you create the Farm and Farm Role. This is where your question was. When creating the Farm, you should select the VPC. When creating the farm the Farm-Role the you select the subnet.
MULTIPLE INTERFACES
In the Farm-Role, look at the "Network" Tab.
SCALR TO CONTROL VPC ROUTING
No, there isn't a way to do that with Scalr. You can build a NAT instance, but beyond that you may want to use a tool like Terraform to control routes, gateway, firewall rules, etc.
GREYED OUT T2.(burstable)
Not all AMIs will run on T2. instance types. You'll need to make sure they are EBS/HVM.
Hope that helps.