The acid treatment isn't required, you can also scarify the seeds in safer ways if they actually need it. I couldn't find more specific help for Sapindus saponaria, but if the seeds are large
enough and there aren't too many you can use something abrasive on each one, the idea is just to make some scratches or sand through one spot so that
water can get through the outer seed coat. If you don't then it's possible the seeds will just need longer to germinate, some seeds like that if just planted want to germinate in the second year. I would try some kind of scarification and then soak them in water for a day, then do regular stratification which is just adding a small amount of damp sand or peat moss to the bag, mix it up and put them in your fridge.
About your other question, I just got some seeds in for Soapwort Saponaria officinalis (which google says doesn't need any stratification, will find out this spring) and it sounds like you boil parts of the plant to make soap.
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