I agreed for an angel to invest $10K even though our minimal is $25K, and now he is holding our funds in escrow until we raise 400K. What should be our response?

You need to just draw a hard line, and say you are not doing rolling closes for less than say $100K (or whatever your threshold is). Explain that you are in talks with investors that may take the full balance, and you are asking investors that want to invest less than -- say $100k -- that they need to remit immediately to hold the spot. Draw a hard line and just push him to make a decision.