The type of light source you use (condenser or diffusion) definitely has an effect on contrast. That's why when you're doing zone system testing you aim for different negative densities depending on which type enlarger head you use. However, once you get your negative exposures right through proper testing, it shouldn't matter which kind of light source you use. Both will provide basically identical contrast with negatives that have been properly exposed and developed for the particular type of light source that you use. If someone has done film speed and development testing while using a diffusion light source, and then switches to a condenser but keeps film speed and development times the same, the resulting negatives will tend to print with greater contrast than he or she was used to. And vice versa with a switch from a condenser head to a diffusion head - the resulting negatives will tend to print with less contrast. Once the optimum time in the developer is reached, additional time doesn't seem to have much effect with today's materials, though Bruce Barnbaum uses an unusually long development time (4 minutes I think), in the belief that it gives him richer blacks. I've tried times up to 4 minutes without noticing any significant difference from my normal time of a minute and a half with Ilford Universal developer and Kodak Polycontrast Fine Art paper. Agitation is important mainly to insure even development and a consistent agitation method is important to help achieve consistency from one print to another. I doubt that it has a significant effect on paper contrast (unlike film) though I've never tried using different agitation methods and observed the results.