So all these changes would need to be done in code:
Add featured images this is an ACF field so you would need to add more via the inc/extras.php file looking for: acf_multiple-featured-image field.
You would then need to update single-projects.php and add in the new image fields.
You can't pick the projects that are featured on homepage it just take the first 6.
For adding a corner flash you will need to use some CSS to get this done.
To add more testimonials you would need to again add this in the extras.php file as this is again an AFC controlled field. You would then need to add these new fields into template-homepage.php.
I suggest making the changes in a child theme so that you can still get any updates to the theme. Thanks.