Instead of trying to reduce contrast with diffusion, you can "pull process" your paper. Increase exposure and reduce development, that will give you softer contrast. It would be a good idea to dilute your developer more than the recommended. The reason is that if your development times are shortish, consistency could suffer. When development rate is slow, you can be pretty accurate and have repeatable results. A side effect of this technique is that print tone can change, giving warmer results. The extent of tonal shift depends on the paper and the developer.