I think the best approach would be to fetch all the data from the DWH, to keep all data, transformation on one place. I have tried several different approaches, but allways found out that there are much simpler to have the data, business rules, transformations rules within the database. The advantage in the long run, is that it will be simpler to maintain. In the prototype phase, I think it would be easier to use the source directly, if you haven't the data in the DW. I know companies using QVD:s as a DW solution as many QV applications are using the same data, to lower the impact of the database load and I think thats a good approach, but the main source to all QlikView applications should allways be the DW.
Keep it simple is a good approach, especially when you are implementing the logic in the client. Avoid using conditions and set analysis as much as possible, especially when you have huge amount of data; try to solve the problems in the LOAD stage or earlier in the DW.
But thats just my opinion.