Actually, to follow up: I just noticed that this page in the XML docs says “InDesign user-created processing instructions are meant for use in other programs; InDesign itself does not act on processing instructions.”

https://helpx.adobe.com/indesign/using/structuring-documents-xml.html#add_comments_and_processing_instructions

Here is another option:
https://www.kahrel.plus.com/indesign/index_update.html

(Jimmy: ID doesn’t expect people to go back and manually tweak all the ranges; it expects that the ranges be set when you first index the document. You’re right that indexing is a huge pain and I wish it were easier!)

Interesting. I have seen that happen and usually it means something has gone wrong and you need to rebuild your InDesign preferences:
https://creativepro.com/resources/faq
Or perhaps some other button has been clicked in the Find/Change dialog box that is limiting the scope.

Jimmy, here’s some ideas:
https://creativepro.com/indexing-a-range-rather-than-individual-pages.php
https://creativepro.com/alternatives-indesign-indexing.php

HI Masood. The only good way to do footnotes for tables is manually (painfully). There is nothing automatic about it. Or, sometimes people make paragraphs that appear like tables, but really are not tables.

There are several alternatives for indexing:
https://creativepro.com/alternatives-indesign-indexing.php

Rivkah: If you choose All Documents then yes, it should find and replace across all OPEN documents. It is very strange that it did not, and I suggest exploring why that is. Note that sometimes you will choose All Documents and then it will change to just “Document” or “Selection” after you search, so you have to change it back.