Verified on Snapshot-2 environment - works as expected: the Error toast notification is displayed in scenario 1, 2 and 3 (see description).
See attached screencast:

Could be tested after UIMARCAUTH-222 will be done

Hi  
About Scenario 4 
Case 1 (works as expected):
When user opens the "MARC Authority" record which is controls "MARC Bib"
AND Delete the 1XX field
Then toast notification "Record cannot be saved without a 1XX field." is displayed.

Case 2 (doesn't work as expected):
When user opens the "MARC Authority" record which is controls "MARC Bib"
AND Edit the 1XX field
AND Delete the 1XX field
Then "Delete fields" modal is displayed AND toast notification "Record cannot be saved without a 1XX field." is displayed.

See attached screencast:

Hi  
I have found a couple issues here:
1) an additional "Authorized" record appears after user edits "1XX" MARC tag - the separate bug was created https://folio-org.atlassian.net/browse/MODQM-305 
2) In the case when:
the user tries to edit "1XX" field (according to requirements, e.g.: from "100" to "110") of "MARC Authority" record which has link(s) but user has opened NOT "Authorized" record - the error toast message doesn't appear. Record successfully saved.
Reproduces for "Reference" and "Auth/Ref" records.
See attached screencast:

3) I have the following question for :
The issue is:
User without permission "quickMARC: Can Link/unlink authority records to bib records" but with permission to edit "MARC Authority" records could successfully edit marc tag value (e.g.: from "100" to 110") in the "MARC Authority" record which has links and error toast notification will not display.
See screencast for example:

Probably it's happening because of failed requests (see screenshot):

Do we need to fix this? For example, we can expand permission "quickMARC: View, edit MARC authorities record" with needed back-end permission to successfully send these requests (see screenshot).

Hey   - For some reason, I cannot retain the styling in the story. See below. 

Scenario 1 Toast notification should display:  Cannot change the saved MARC authority field 100 because it controls a bibliographic field(s). To change this 1XX, you must unlink all controlled bibliographic fields.

Scenario 2 Toast notification should display: Cannot add a $t to the 100 field because it controls a bibliographic field(s) that cannot control this subfield. To change this 1XX value, you must unlink all controlled bibliographic fields that cannot control $t.

Scenario 3 Toast notification should display: Cannot remove $t from the 100 field because it controls a bibliographic field(s) that requires  this subfield. To change this 1XX value, you must unlink all controlled bibliographic fields that requires $t to be controlled.

Scenario 4 Toast notification should display: Record cannot be saved without a 1XX field.