Profile: Yidna

Yes that is the corerct location

source\Presentation\Nop.Web\Views\Shared\_Root.Head.cshtml

After changing you need to restart the webserver / application

What version are you building ?
You can look at other plugins with data access i.e. Nop.Plugin.Shipping.FixedByWeightByTotal
or https://docs.nopcommerce.com/en/developer/plugins/plugin-with-data-access.html

Not sure
You can limit a product availability to a customer role
So it follows that the Attributes defined for that Product are limited to that role

Maybe you would need to modify or override
source\Presentation\Nop.Web\Views\Product\_ProductAttributes.cshtml

Is there a reason why you did not use Product Attributes which are stored Automatically ?
If you used Prodct Attributes then it would be automatically displayed on the product page, in the cart and in the order?

What does the DropDown value represent ?
If you want to display the Drop down value in the Cart from a plugin then you need to make an override and new a new razor page.
If you want to store the data you can either make a new table or store it in order.CustomValuesXml

Yes running in Visual Studio that's how it works
Just restart IIS Express after Restart Application or Plugin Changes