here's the solution I went with:
esxcli storage vmfs snapshot list
Volume Name: (name of datastore for parent volume of snapshot clone volume I'm trying to mount)
VMFS UUID: **
Can mount: false
Reason for un-mountability: duplicate extents found
Can resignature: false
Reason for non-resignaturability: duplicate extents found
Unresolved Extent Count: 3
verified hba numbering
esxcli iscsi adapter discovery sendtarget list
Adapter Sendtarget
------- ------------------
vmhba33 **:3260
removed all iscsi devices + sessions for multiple snapshot clone volumes associated with the same parent volume on the san array:
esxcli storage core device set --state=off -d eui.**
esxcli iscsi session remove -A vmhba33 -n iqn.**
esxcli storage core device set --state=off -d eui.**
esxcli iscsi session remove -A vmhba33 -n iqn.**
esxcli storage core device set --state=off -d eui.**
esxcli iscsi session remove -A vmhba33 -n iqn.**
esxcli storage core device set --state=off -d eui.**
esxcli iscsi session remove -A vmhba33 -n iqn.**
esxcli storage vmfs snapshot list
(no longer lists it)
re-added only the device / target for the snapshot I'm trying to mount with target + serial verified from san array:
esxcli storage core device set --state=on -d eui.**
esxcli iscsi session add -A vmhba33 -n iqn.**
The mount + resignature options are no longer grayed out in the vsphere client's Add Storage screen and the volume was successfully resignatured + mounted.