I think dilution affects the contrast too, but never tried it. With Rodinal I experienced a change in contrast linked with temperature (higher the temp, higher the contrast, with Orwo UN54 I got the same evolution). If you want to keep with Rodinal and the contrast is low, try higher temperature, like I already mentioned.
As for the halide solvents (KSCN or Na2S2O3), I was always told that with Tri-X they weren't needed. When I tried to add some to the 1st developer, it lowered the density overally so I decided not to use it. There's a huge debate on whether to use or not to use the silver halide solvents in first developer, so I guess you have to try and decide your own...