You cannot change the priority of indices.

You can ask IRIS to ignore particular index -- %IGNOREINDEX, or disable index for a particular condition -- %NOINDEX

You can force join order with %INORDER hint

https://docs.intersystems.com/iris20253/csp/docbook/DocBook.UI.Page.cls?KEY=GSOC_hints

Generally, if you see that some query does not use particular index, that should be used, or chooses bad join order -- collect table statistics, if this does not help -- contact InterSystems Support (WRC), or post details here -- the query, its plan, and describe what plan you'd like to see

If the CSPGatewayLatency is almost 5 seconds -- ignore, this is false positive, fixed in 2024.1.2. Or better -- upgrade! ;-) https://docs.intersystems.com/iris20241/csp/docbook/changes/index.html#CSPServer_DP-433569

No.

You can filter results based on the row level security policy Row-Level Security https://docs.intersystems.com/iris20252/csp/docbook/Doc.View.cls?KEY=GOBJ_persother_rls#GOBJ_persother_rlssetup

You can also grant access to specific columns in tables to particular roles, and then grant these roles to a particular user https://docs.intersystems.com/iris20252/csp/docbook/Doc.View.cls?KEY=RSQL_grant#RSQL_grant_synopsis