I think you are very wise to ask these questions, and just b/c you "have" the money doesn't mean you have less of a reason to ask them!!
1. I think your SW is wrong that all domestic adoptions cost 20 - 25K, I have heard of many stories where the adoption didn't cost that much! Ours was half that (AA).
2. I would tend to think that if your SW has been professional and you like the way she handled your homestudy that it is a good thing she is well known at the agencies she recommended.
3. Did you look at the thread on this forum where people recommend the agencies, etc. that they have worked with - that is a start. Also, are there any adoption support groups in your area, maybe attend one and talk with some of the people there.
4. I am not too sure how many agencies will give you the accounting that you are looking for, ours didn't - but we were ok with that. As to your last question of how do you keep from being taken - well I think you are on to a great start by asking these questions from the beginning. You have to be cautious, but follow your heart and your gut - those instincts are usually correct.
Good luck to you!!