I would not presume I know anything about your area in the first place.  But your questions caught my eye.  Persistently wet soils are typically wetlands, and in our country we have lost many of them to farming in the past.  Wetlands tend to have a multitude of "potential benefits" for plant and animal habitat, reduced flooding, potential to store or retain pollutants, etc.  Early settlers tended to drain these areas for farming and other uses, and now there is increased consideration of these areas including restoration for habitat and hydrology reasons.  So please consult your ecology and wildlife biology people.  If you persist, the best time probably to mix these soils is probably during a drought.  If too wet, they will probably puddle, loosing almost all soil structure as a growing media.  Mixing the soil does not necessarily mean that during normal periods they will be dry.  Some soil materials and circumstances are very hard to change, and I would not pretend to tell you what they are.  If the area has a high water table, soil mixing is probably not going to make much difference.  In our coastal areas, pine trees use in evapotranspiration of up to 1 m of water per year, so planting and growing vegetation that need a lot of water may help dry out areas.  Farming will probably reduce water use by plants, so the area may be wetter than it is now if currently forested.  If the goal is to get water through a hard pan, and the pan is shallow, you might try ripping to a depth of about 24 inches under dry conditions so the soils are not plastic.  It is sometimes better to put the rippers on the front of the dozer for greater push, rather than trying to pull the rippers through the soil. 

To my knowledge, one of the measures to determine the workability is slump testing which is often used prior to the concreting of diaphragm wall or superstructure. By the slump testing, you will be able to see whether the concrete adopted can flow far enough.

I found in a french document, that the Atterberg liquid limit wL is used to determine the water ratio needed for a good workaility.

However, it dosent give a mixtures that flow.

Do you mean by workability the plasticity potential of the mixing soil elements!!

In fact, it's rather liquidity than plasticity. Workability for wet soil mixing can be considered like the flowability of self compacting concrete (SCC).

It's indicated by the amount of useful internal work required to fully compact the soil mixing without bleeding, generally with no requirement for vibration.

The main factors affecting this property are:

Slump test