Problem:

Will using Toolset on both dev and main site count as 1 or 2 site licenses?

I want to keep the auto-updates going on dev as well.

Solution:

You can install the Toolset Types plugin, and click the link "This is a development / staging site", so you can still get auto-updates in the development website

Relevant Documentation:

Problem:

The issue here is that the user wanted to edit a single custom field on their posts in a list without going to the post page itself.

Solution:
To resolve this you can create an edit post form and set it to submit using AJAX. Then removed all the fields that aren't needed leaving only the needed field and the submit button.

Finally just add that form to the view listing out your posts. From there you can Edit the Field A and change the values for each of the posts without having to go to the post itself. You can edit the fields from the list.