Look into "Application Initialization" IIS 7.5, Windows 2008 R2 (harder to setup) IIS 8, Windows 2012
App Initialization allow any application (application pool not site) restart to overlap and use the old, still running the previous application while warming up the new application start. Once the new application is spun up (determined by URLs you can set),...
There are a number of ways to handle what you're asking for, and a few different aspects to your question:
Handle small updates for promotions
What you're really after here is a content management system or similar that allows you to edit the content on the fly (think Wordpress/Drupal or from a .NET point of view N2 CMS, Umbraco, Orchard, etc.), however ...
I recommend you use Googlebot's fetch and render tool. It will show you exactly how Googlebot sees the webpage with your resx files and whether or not the resx script has an impact on Google's crawling service.
If you're going to use friendly URLs (which a fine idea), go all the way. Make sure that all links on your site use them, including the sitemap.
Then, as Stephen touched on, either use 301 redirects or canonical tags so that if any crawlers visit your non-friendly URLs, they are pointed to the canonical page.
ASP.NET Event validation does provide some level of protection against some specific web security attacks.
Request Validation, a feature included in ASP.NET 1.1, may not adequately sanitize hostile user-supplied input. ASP scripts that depend on Request Validation to sanitize user-supplied input may still be prone to cross-site scripting or HTML injection ...