They will probably have an impact on response rate which will further complicate your analysis when comparing results. The lower the response rate the less sure you can be of your results.

How will you adjust for topic? If its an IV then the regression (if that is what you are doing) will show you if its a signficant variable and having it in the model will control for its effect. I am not sure what interaction term you are suggesting here.

Your third solution seems best me off hand. Again if topic is a predictor (rather than something you predict) placing it in the regression should correct this problem. What it won't do at all is bring you the same response rates - ones with more interest will likely generate a higher response rate.