I forgot to clarify previously... yes it would have been more appropriate to post this in the Dev Edition forum. We actually prefer customers posting in the forum for the product they are actually using, even coding questions like this one.
Now back to your specific test scenario.
Correct, presently each test (both master and subtest) can be bound to only one explicit data source, or use the source of the parent test. I should explain a little how things work under the cover so we can successfully achieve the goal you are seeking.
When you bind a test to an Excel spreadsheet, the tool actually makes a copy of the file and places it into the Data folder that is contained in your test project folder. Thus in the end all data bound tests will draw from files contained in this specific folder. As a result, even relative paths won't work like you envision.
When a test starts it reads all of the data from the datasource, creates a .NET DataTable object out of it, then starts running the test iterating through the rows of data.
This is true for subtests as well. The good news is that the subtest doesn't read its data source until that subtest gains control. So while the parent test is executing, if it can adjust the data in the data source (e.g. swap out the physical file in the Data folder) for a subtest its about to call you can achieve the effect of parameterized data source for your subtests. Using your relative path and filename idea in a column of the parents test can be used here. Have a coded step that takes that string and performs a copy from that path to a static filename in the Data folder actually should work.
On another of your questions, unfortunately, no, data values cannot be used in one of our IF ELSE condition blocks. It will only accept element verifications at present for the IF condition.
the Telerik team