1) How can I perform an optimization based on only a subset of the response variables (e.g., 28 d strength and slump)?

Simply set the weights for the response variables in which you are not interested to zero. You may do this from the "Change response variable limits, weights, and function types" link at the top of the Analyze the Data page of the COST system. Then, rerun either step 7 or step 9 to determine the new optimal settings for your new optimization criteria.

2) I performed a first analysis using COST, changed a few parameters and redid the analysis. But, the graphs appear to be the same as in the first run. Why is this?

Some WWW browsers do not update gif images if they already exist in their cache. Pressing the reload button on your WWW browser may bring up the new gif graph from the COST system. But, the surest way to remedy this problem is to exit the WWW browser and restart it from the operating system (then reperform the analyze data function).