I ran across this problem last year. The issue at hand was disproving that the Zone B* as depicted was an area of 100-year flooding subject to shallow flooding, and the reason it had to be disproven was that a local bylaw prevented the inclusion of land subject to 100-year flooding from the...
What type of inlet? If you are using a cast grate, the foundry will have pass flow tests available for your use. Often, they test the grate at different slopes, so the value you seek may be available.
In my experience, using the TR-20 rainfall as the Intensity value in Rational flows will result in a lower runoff. At least in my area (Massachusetts), the IDF curve will yield 'I' values approx. an inch higher (for short Tc's). Remember, 'I' values vary with Tc. Why not accept the TR-20 flow...
Links would work fine for upstream contributory flow, just remember to calc the other project with the correct rainfall for each storm before accepting the flows into your model.