Hi Ole
You have 3 questions here
1. adding placeholder text
2. having the search form submit when hitting enter
3. styling the inputs
For the record I will describe what is required below, but skip to the end to understand why this is not working.
1. You can use JS to set the placeholder attribute on the form text input
I tried the solution in the first of your links and it worked okay, so let me repeat it here.
In your search view where you have added the [wpv-filter-search-box] shortcode, add the following to the custom JS box:
jQuery(document).ready(function(){
jQuery("input[name='wpv_post_search']").attr("placeholder", "Search here");
});
Edit this to replace the placeholder text as desired.
2. It should do already, it is standard HTML form behaviour, and it does on my test site. It doesn't matter what you views settings are.
3. To be honest, I couldn't quite understand what your issue here was, but you can add classes when inserting the shortcodes to style the inputs, and when you add the shortcode for the submit button you can specify whether you want the markup to be an input or a button.
Why isn't the above working?
Because you have inserted the shortcodes into your page layout outside the context of a search form. You need to create a custom search view and insert the text search and submit inputs as filter controls in that view.
From your link it looks like you already have the search form in the sidebar. Your text search field needs to be part of that search form.