I have something similar, which sorts based on subcategory position and also sorts based on attribute value position (not attribute value name). Mysql it works fine, ElasticSearch - I've tried using using both afterSetCollection and aroundSetCollection, however I have been unsuccessful at reordering the ElasticSearch results. Did you get any futher?

You have to make sure you have both the execute and the massAction functions in the controller

You'd have to think about it the other way round. So if treat the product price as 0 - but you customise it to add $200. In this case you would add 0. But your other options may add $200 Example. Product price: 0 Add Installation: $200 No Installation: 0

Impossible to know, without knowing what block your custom.phtml is using. Is the product / item loaded?

I wouldn't get too hung on it. Life goes on and people need up to date answers. Moving on

Poster just said M2... Anyway, 2.2 - 2.4 should be fine. But tested on 2.3 - 2.4. 2.2 was just terrible branch of M2 and hopefully everyone has upgraded to 2.3 now.

@Typo - No problem, I've added another solution to this answer for a quick fix version

To remove that block, you just need to do <referenceBlock name="customer.new" remove="true"/>