As far as I know, to answer various points of your question as below: -
1. IF all the nodes of a NSXT manager cluster fail or go down, all your existent policies will still remain but you will not have any UI or API access to the NSX-T setup. You cannot perform any sort of operations(either creation of new objects or update to existent objects or deletion of objects, so no operations are possible). Your VMs operating in the dataplane will still continue to operate in headless mode till a new NSXT manager node is deployed or the older ones are restored. So your VMs would not be disrupted,but you cant do any operations or modifications in your dataplane.
2. Since the NSX-T is a unified appliance(since NSXT release 2.4) with all components like Manager, controllers, policy etc all in the same appliance, so with the failure of all nodes of the NSXT manager, you would have lost your Management and Controller planes, and this is the reason why the user cannot perform any operations on the NSXT setup, and the dataplane will continue operations in a headless mode till atleast 1 node is restored or new one deployed.
3. To answer this part, from the NSX-T versions of 4.x the NVDS hostswitch type is deprecated and only VDS option would be available for the user. With the VDS option, all networking features of the regular VDS(like PVlan, netflow, portmirroring etc) would be available to use in integration with NSX-T, which would not have been available with NVDS. Besides VDS logic is already integrated in the vCenter, so this makes it possible to use all the vCenter API's for operations on the VDS which would not have been possible with the NVDS. So ease of administration and management is another advantage of moving from the NVDS to the VDS. These are the reasons why VMware is eventually moving away from the NVDS and has deprecated it from NSX 4.x releases.
I have tried to answer all the questions to the best of my knowledge, and hope this helps and you find it useful. If so you could mark your query as resolved and please give a kudos.