Difference between index catalog_category_product and catalog_product_category

Just add this line to your .vimrc:

set showcmd

From the Vim help:

Show (partial) command in the last line of the screen.  Set this
option off if your terminal is slow.
In Visual mode the size of the selected area is shown:
- When selecting characters within a line, the number of characters.
- When selecting more than one line, the number of lines.
- When selecting a block, the size in screen characters:
  {lines}x{columns}.

Do I need separate orgs for development and packaging?

No, but it is strongly recommended. For today, ISVs use unmanaged Development Edition orgs. In DX, you'll be using throw-away orgs for development, and a primary packaging org to distribute your app. You need one packaging org per managed package.

Do the orgs need to remain the same through out the life cycle of the app? Or can they be changed after the security review?

Your packaging org will always remain the same for all time. Your development orgs are not relevant for your security review. For the security review, you'll need a testing org must remain unmodified until the review is complete, including pushing new versions. If you do so, it will not invalidate your security review, but it will delay your security review.

What orgs do you need for applying for security review?

Your packaging org and a demo org with your package installed. This is outlined in the security review process once you apply.

What is the process for development for different org types and releases?

You only package in a single primary org. In the current ISV cycle, you use ISV Developer Orgs, and use migration tools to move code and metadata into the managed package. In the near future, you'll use DX scratch orgs to do your development. In DX, you can build Professional Edition and Enterprise Edition Developer Orgs so you can develop in either environment, as well as test how your code works in each edition.

Take a look at Skeletron, which is a Python tool to merge adjacent lines.

(Incidentally, I came here via your request in #osm IRC, but it would be polite for you to wait there a little more than 30 seconds to see if someone has an answer to your question!)