I have a 2000 Volvo S80 2.9. If one bottle does not stop the leak, would adding a bottle be ok for my cooling system without a radiator flush and antifreeze drain?

If one bottle does not stop the leak, unfortunately you may need to replace the leaking part.

Does FCP Euro: 1- recommend this product to seal antifreeze leaks from Head Gaskets? 2- If not, what product brand do you recommend? 3- Carry any other comparable product?

This product only addresses small leaks. Anything that can't be solved with this product is considered a major head gasket leak. We only recommend this product as it does not clog passages or create build ups within the cooling system. If this product does not help with an existing leak we believe the best solution is to perform the proper repair and replace the head gasket.