When we create a folder in the SuiteApp directory and launch an HTML file it seems to accept relative URLs in the code just fine. However, when the client account does NOT have SiteBuilder or SCA it seems to fail miserably. For SuiteApps, I think this is unacceptable. Am I wrong? Should we be able to expect consistency? How hard would it be to allow this in all accounts?

Consistency? From netsuite? They don't even expose "lineuniquekey" on all transactions despite it being the primary key in the transaction database