Hope this helps:
1. the nsx-t support multi-VC environment? Yes, but it's a little different than NSX-V. A single NSX-T manager can support up to 5 vCenters (as opposed to the 1:1 ratio for V) but you can't federate multiple NSX-T managers yet like you can with V.
2. does it support automated deployment?(such as deploy the controller, edge, router VMs) Yes. Like V, any functionality available in the UI is available in the public API.
3. it can be used in "environments which only have KVM" therefore it can be treated as a SDN product for KVM? Yes. There's a decent overview in the Working with KVM section of the install guide.
4. will the logical switch utilize vxlan in the future? or it only utilize GENEVE? Just GENEVE today, not sure about the future.
5. does the value 1600 suitable to MTU or 2000 even 9000 are suggested? 1600 is still fine, but like NSX-V, larger MTUs result in slightly better performance so I'd still go 9000 bytes if you can.
6. does the "tiered router" better than the "distributed router"(nsx-v)? While it is architecturally different than NSX-V, there's still a distributed routing component for both the Tier 0 and Tier 1 routing constructs so you don't lose anything performance wise. Check out the NSX-T: Routing where you need it (multi-hypervisor & multi-cloud) blog posts for a more in depth description.