Q1) Can we show specific message in a specific page in our website. How to select this place where message will be visible??
Yes, you can set up a message on different pages.
You do this by Opencart - Design - Layout system.
So you can select which layout you want to show like home, category, product etc.
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You can also show the message on specific product or category.
Q2) Hi, I would like to use this on the success page to let customers know where their confirmation email has been sent e.g: "Thanks for your order, your conf email has been sent to (Dynamically insert customer email here)"?
Yes, you can. Enable the message for a customer group. So only logged in customers can see them.
And then show this message via Design - Layout on your checkout/success page.

Make sure to add to add this tag in your message {customer_email}
Q3) I have added a wide image to the home page using this module but the image does not resize responsively. Any chance?
When you create a message in backend.
Click on code view button on the editor.

You will see :
<img src="path">

You need to make it here:
<img width="100%" src="path">
Q4) Is it possible to display a message in search results page? My intention is to display a message like: 'Νot found what you're looking for? Contact Us'?
Yes possible. Create a message and display it on product/search page using Design - Layout - Search page.
Q5) will this work with the custom theme? Ex journal,fastor?
Yes, it supports all themes.
Q6) my site using both French and English. Does this support multi language or will I have to put both languages on one message??
Yes, it supports Multi-Language. You can display messages in all your languages.
Q7) Can we show a message on specific product and all products together??
Yes, you can.
Using Design - Layouts you can show the message on all the product pages.
And if you select the specific product on the message settings, then it shall show on that specific page.
And both options can be done together.