1. If the hubs are in the same organization and you use Auto-VPN the hubs will automatically build AutoVPN tunnels between them self. This is the default hub behavior.
If you want the connectivity between cloud providers to be handled in Azure and AWS i would do VPN tunnels with the VPN gateways. This would be more complicated then the above option, but might save on cost or capacity depending on your vMX size and billing.
2. Yes, but i would use hub and spoke. Full mesh means that all hubs and spokes form direct tunnels with each other. This is hard to scale unless you have a low amount of MX appliances in your org.
I would do hub and spoke. Add both the AWS and Azure vMX appliances as hubs in your topology. Your spoke site will then create auto vpn tunnels to both.
MLL