People make that kind of table all the time. Attached is an example (V 9.3) from the previous question I looked at.
I'm concerned about your statement about manual entry. There is no provision for manual entry of data in Tableau.
Test 2.twbx 1.2 MB
I'll anxiously watch to see what others have to say, however my work with Tableau has shown me that the manual entering of data is difficult. User interaction is usually limited to filters and parameters.
If I understand you correctly, I think your best bet would be an Excel worksheet (or Access form) for the data entry, feeding into Tableau for display,
I'm a flag-waving Tableau user: and while Tableau rocks when it comes to visual analytics, if you're not using this strength, another platform might be a better option.