Shuming,
For population-level analysis, it would be more informative to focus on all time points rather than extracting just one or a few. In other words, consider using all the estimated effects (i.e., sub-bricks labeled *#0_Coef, *#1_Coef, ..., *#6_Coef) to capture the full temporal profile. You can refer to this blog post for further insights.
Regarding your question about the last two time points in your design matrix, I can’t provide specific guidance without knowing the stimulus timing for the final trial. I suggest using 1dplot to visualize the last segment of the seven regressors for that particular condition. This can help you identify any irregularities or patterns that might raise concerns.
Gang Chen