13 Can I have a workflow on recording the correct set up .
14 How do I save it as a script
15 How do I apply that script
16 What is a script
Ok answering your last question first.
A script is a small program which carries out a series of repeatable steps. (In Photoshop it is called an action) .
Some things can be included in a script because they are the same every time; others cannot eg cropping where you might crop a rectangle in top left in one image, centre in another and bottom right in a third or cloning a tree out of a landscape picture . The script cannot know what picture you are working on and cannot crop or clone intelligently.
You can bring up the script toolbar as shown:

Some of the parts of the script toolbar are:

To prepare for a script recording you need to do a dry run with an image to familiarise the steps you are taking. Then you start your recording by clicking the "Start Script Recording" button, then do the steps:
1. Image -->Resize (height 900 should already be set and remembered), press OK
2. Image --> Canvas Size (width 1600 should already be remembered and all controls including colour of size bars remain) , Press OK
3. Click Save Script Recording and type a save name to remind you what it is (I used landscaperesizeheight900canvas1600) . It will try and save it in the Scripts Restricted folder , but go up a level and choose to save it in the Scripts Trusted Folder. Click Save to complete the save of your script.
Now you can test your script to see if it works as planned.
Bring in a new landscape image. Choose the script you have just saved in the Select Script box. (click the little down arrow and scroll down in the drop down box to the newly saved script.)
Click the "Play Chosen Script" button and the script will start on your image. There are two ways it can operate - Step by step (Interactive mode - high lighted button with border) where, as each dialog box comes up you have to Click OK
OR
It goes right to the end (silent mode) and you finish with the image and the side bars at 1600 x 900.
Have done some tests and you will have to separate your images into landscape and portrait.
I took some landscape images and resized them to different heights (all more than 900) and this script resized them all correctly and placed them on the 16 : 9 canvas. However when this script was applied to different portraits it failed. So you will need one script for landscapes and then repeat a different script for a portrait image. In processing your images you will need to separate your landscapes and portraits temporarily to apply the respective scripts.
This shows some results if you apply the correct scripts.

And if you try a script for the wrong orientation.

Next The Batch Process.