I can open all .pack files of ETW with the latest version of the
PACK FILE MANAGER 3.5.2 without any problems.
As you see in my
AUM-EMP I used two types of database tables to add custom or vanilla units to the related faction or faction groups:
aum_units_to_exclusive_faction_permissions
aum_units_to_groupings_military_permissions
To allow or deny units for a certain faction you can use a custom
yourname_units_to_exclusive_faction_permissions db table and then add new rows to it.
The third column is called "allowed" and there you can "activate" or "de-activate" the units which you want or which you don't want for the related faction.
In your case you can "de-activate" all artillery units from the Native American factions like "cherokee_playable, huron_playable, iroquoi_playable, plains_playable, pueblo_playable".
The database table
units_to_groupings_military_permissions works a bit different. Basically to remove assigned vanilla units in this table, you must create a copy of this vanilla table with the complete content and call it also
units_to_groupings_military_permissions in your own mod. If the same db table name is used like in vanilla, then your own db table overwrites the vanilla table completely. Then you can remove the unwanted rows in your custom table.
In your case you should look for the amerinds_group.