Good questions all. Generally speaking bi-bi education, to be successful in the way it is intended, does require a commitment from the child's family to communicate in sign language. The idea is that ASL (in this country; of course there is bi-bi education in non-ASL-using countries but I'm going to stick with the U.S. for this) is a deaf child's natural first language, and he should be able to acquire it the way hearing children acquire their first language -- naturally.
(Studies supporting this have shown that when deaf children are raised by native deaf signers, their language develops in almost the exact same way as hearing children; i.e., it doesn't matter whether the language is signed or spoken as long as it is acquired naturally. This is usually easier with ASL and deaf children but of course not in all cases.)
So having learned sign language naturally, the deaf child is then in a good position to learn English as a second language because he can transfer what he knows instinctually about language to the second language. This is a very, very important point about languages: it is much, much more difficult to acquire other languages when a first one has not been acquired by the "critical age" (usually around age 12, I think?)
This is one problem with bi-bi education: many children going into it did not fully acquire ASL, usually because they were the only deaf child in a hearing family that did not commit to learning and teaching sign language. So people point to failures of bi-bi education as a problem with the theory, when really it is a problem with the inconsistency of the language that children grew up with. Another problem is that programs that call themselves bi-bi often do not employ staff who are fully proficient in both languages. So children do not get good role models in one or both languages. This is a problem with the implementation of bi-bi education, not the theory.
I know I rambled a bit and didn't answer all your questions but hopefully some of this is helpful.
