Hey all, thanks for the feedback.
I ended up opening a case with Microsoft, asking them about the requirement for different instance names. After some back and forth, Microsoft has confirmed that the requirement for different instance names does indeed apply in the scenario of 2 Failover-Cluster Instances (FCIs) on 2 different subnets, as part of the same WSFC cluster.
In my case - one FCI on subnet1 and one standalone instance on subnet2 - the instance names can be the same;
Microsoft supports this configuration.
I hit a couple of snags, however, while configuring this setup:
(1) While installing the SQL standalone instance on subnet2 I got this error:
The directory d:\xxxxxxxxx\xxxxx is not a valid path. Standalone sql server on wfc cluster cannot have files on shared disk locations
The remedy for this was to evict that node from the WSFC cluster, do the SQL install, and add it back:
(2) Error while creating the Availability Group:
Failed to create, join or add replica to availability group 'MYAG1', because node 'MyNode3' is a possible owner for both replica 'SRVS1\Instance1' and 'SRVPH1\Instance1'. If one replica is failover cluster instance, remove the overlapped node from its possible owners and try again.
The cure here was to go to Failover Cluster Manager and unselect the 3rd node, SRVPH1, (where the standalone SQL instance was located) from the list of possible owners of the FCI.
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