First I try to address b)

Can external segregation (i..e, tunnel type between compute node and network node) be defined independent of internal segregation and what are the configuration variables controlling that?

via thread
https://ask.openstack.org/en/question...
Per daragh-oreilly :-
If tenant_network_type=gre then when a tenant creates a network they will get a GRE backed network. But the provider/admin can still create VLAN backed networks by specifying the provider attributes when creating the network with the CLI.

neutron net-create net1 --tenant-id $TENANT_ID --provider:network_type vlan --provider:physical_network vlan_net1 --provider:segmentation_id 100

The last command will succeed if in ml2_conf.ini type_drivers=gre,vlan
So ml2_conf.ini may contain

type_drivers = gre,vlan
tenants_network_type = gre,vlan
tunnel_type=gre

You stated

Initially I thought the ML2 tenant_network_type variable defines the type of internal segregation (VLAN, GRE, VXLAN) for networks belonging to different tenants using the same compute node (i.e., the network between the VMs and br-int)

I think that

Tenant_network_type defines actually type of communication between Compute and Network Nodes  for all tenants on the system.  Addressing b) is showing that even if  tenant_network_type= vxlan ( or gre ) in ml2_conf.ini.  Tenant's network of VLAN type might be created by admin.

Second a) by default the following is supposed to happen ( i am unaware of different schema )

As far as to my knowledge both VXLAN and GRE transmissions leaving Compute node get local vlan tag stripped and  tenants VXLAN(GRE) ID assigned. At arriving to Network Node VXLAN(GRE) ID will be stripped and local vlan tag   assigned and vice/versa. This vlan tags are only localy valuable to split traffics on local br-int. Globally over all landscape VXLAN (GRE) ID obtained by tenant  identifies his private subnet.