>How much lower is this temperature?

Here is the reactivity as a function of temperature for a few candidate reactions.

>Is the main motivation of using Tritium the lower temperature or actually the breeding reaction?

The main motivation is the temperature. Obtaining fuel for DD is not an issue, because there's plentiful deuterium in nature (seawater, for example). It's a nice benefit, and quite important for tritium, which is not found naturally in large amounts. We have to produce tritium somehow, and having the reactor breed its own fuel is a nice way to do that.

>How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

Not sure what you mean here.