Profile: Yidna

There maybe another way to do it ?

But unless you want to change the core then you need to go up a level and override a method in ProductController that calls IsAvailable
nopCommerce_3.90_Source\Presentation\Nop.Web\Controllers\ProductController.cs
Then either create you own IsAvailable or use the current one in conjunction with your extra controls

you can insert the script in the page or use a Widget
See source43\Presentation\Nop.Web\Views\Catalog\CategoryTemplate.ProductsInGridOrLines.cshtml

Do they have a SDK ?
Is there a website which shows what / how to send ?

There are some editor settings maybe try changing them
Goto All settings
Search for richeditor
adminareasettings.richeditorallowjavascript
adminareasettings.richeditorallowstyletag

Did you try and restart the webserver ?

To fix the error copy the backed up version (distribution version) of the file back to your server

Is it the only thing you changed ?
you probably need to copy other .dlls to the server that reference this one i.e. Nop.Web