An ASP global.asa file will be out of the scope of ASPX pages. They operate in different application spaces with different programs. The .NET global.asax is in the scope of ASPX pages.
Instead of the ASP syntax using <% %> you should be able to get away with something like this:
<%# Application["MY_VAR"] %>
Remember, if your page language is C# you'll need to use the right indexing syntax : [n] versus (n). That's probably what causes the first error.