It sounds like you need to build a big list of all the things you're not certain about AND all the assumptions you've already made, and shop them around the peo

It sounds like you need to build a big list of all the things you're not certain about AND all the assumptions you've already made, and shop them around the people who DO have the domain knowledge, and ask them to "make sure all this makes sense", be up-front and humble about your lack of domain knowledge, and you should do just fine.