Decision tables in the database: a valid solution?
The additional module will be used by no more than 20 people on an intranet.


"People" is a fuzzy term when you're talking about legal applications. Supporting 20 lawyers is a lot different than supporting 1 lawyer, 4 paralegals, and 15 staff.


  For legal cases new business rules are more like "everything is an
  exception"


And that's one reason why. Within case management, certain kinds of things are simply deterministic. Timing of responses, deadlines for filing, things like that. But the rules that govern were written by lawyers for lawyers. Companies that build case management software have lawyers on staff to read the rules for procedure (which change over time) and to verify that their software complies with the rules. 

Some of them have a lot of lawyers.

Other kinds of things are governed by whimsy. For example, some case management packages let users "template" their tasks. Entering something like "Deposition" on the calendar might also calendar a series of earlier tasks that have to be done to prepare for the deposition.  Under the pressure of multiple, incompatible deadlines, parts of a task chain get dropped or given short shrift.

Now, to (finally) get to your question . . .

Decision tables in the database are a viable solution. In fact, decision tables constitute a database whether they're stored in Oracle or in application code. There are obvious trade offs to storing decision tables in application code.

But decision tables in the database are probably not a viable solution for you. The problem you have, if I understand you correctly, is that you think you might need to simplify the architecture to decision tables in order to eliminate (or at least to greatly reduce) the use of Microsoft Workflow. With just two of you and a minimal budget, success will be difficult at best.

I can suggest two things.


Can you economically produce a prototype of a replacement system
built on decision tables?
Can you use better hardware on the existing system?