Hi Nancy!
I nevertheless shall try to convince you. I bring a piece of the program (to answer all your questions), which spoils an index. I hope it will convince you, that the index can be broken in the program.
I apologize for the long text of an example.

CLOSE database
SET safe off
CREA database d
CREATE table a (key c(3) primary key) && To create the table with an index
FOR i=300 to 310
INSERT into a (key) values (str(i,3)) && To fill in the table
ENDFOR
nn=recc() && remember amount of records
USE
new_rec='999'
form1=createobject('form1') && SHOULD BE 1. with a flag readonly
form2=createobject('form2') && add new record
Release form1,form2 && the forms can be closed
**********************CHECK of RECORD**********************************
USE a excl &&
if recc()>nn && There is a new record
go bott
wait wind 'The new record is found | '+key nowait
SET order to key && Search on an index
IF !seek(new_rec)
&& By the way, the record is not visible and by search LOCATE
=messagebox('The index has failed')
set order to
brow nowait
if messagebox('REINDEX ?',4)=6
&& It does not change index expression, the filter and does not add record
reindex && To restore an index.
set order to key
IF !seek(new_rec)
=messagebox('The record is not found')
else
=messagebox('Now record is found!!!!!!!!') && However, the record occurs
endif
endif
ENDIF
endif
RETURN
************************NECESSARY CONDITIONS*****************************
DEFINE class form1 as form && FORM 1
DataSession=2 && PRIVATE
Name='READONLY'
PROCEDURE init
USE a noupdate && READONLY!!!
ENDPROC
ENDDEFINE && These definitions in the form 1 - SUFFICES!

DEFINE class form2 as form
DataSession=2
Name='WRITE'
PROCEDURE init
USE a
SET multi on
CursorSetProp('buffering',5,'a')
INSERT into a (key) values (new_rec) && new record
BEGIN transaction
ff=tableupdate(.T.,.T.,'a')&& update
IF ff && ready ?
END transaction
WAIT wind 'Normal end of a Transaction'
ELSE
ROLLBACK
=messagebox('ERROR')
ENDIF
ENDPROC
ENDDEFINE

Well here and whole example.