"Bilingualism and biliteracy are not separable. Literacy is a pre-requisite in modern society for having sufficient command of a language to learn it to a functional standard and to operate in it in adult society."
I really think you need to define what you are talking about, Anna. You joined this thread apparently taking a stance on all experiences of bilingualism - yet this remark clearly excludes cultures where the language is rarely written down (Australian aboriginal languages); languages where significant proportions of native speakers are illiterate or partially literate (certain Indian languages, for example); languages where literacy is as culturally determined as the spoken language (varieties of French, mainland v Taiwanese Chinese scripts); languages where literacy is contextual (different scripts and language forms for religious use in Russian and Arabic.)
"Adult "bilingualism" (I don't want to use that term) without adult biliteracy is only marginally advantageous." Advantageous to whom and for what? For white-collar jobs, maybe; for speaking to your extended family, not so.
The viewpoint you now seem to be representing is that "proper" and "worthwhile" bilingualism is in two high-status languages for use in middle-class, professional contexts.
The bottom line is, bringing up middle class children costs money, however many languages they speak. That actually seems to be a lot closer to the view you are espousing than anything to do with the genuine concerns of bilingualism.
As for my experience - you are right, I have not been attempting to bring up a bilingual child for long. But the reason I chose to attempt to do so is that I have an immense amount of knowledge and experience in language learning and teaching and the nature of bilingualism to have an idea on "strategies for bilingualism". I will have to test them - as, presumably, you will. But I resent the implication that my ideas have no basis in fact, theory or experience.
Here are the facts. There are thousands of Bengali adults (to pick a group at random) of my age in this country who come from relatively humble backgrounds. It is nlikely that the majority had "bilingual schooling, frequent trips to [Bangladesh](with summer camps etc thrown in), books, [videos], cinema etc (and to hang out with lots of affluent international families)" and, at that time, support in schools was even poorer than now. Yet many of these adults are functionally bilingual in English and Bengali and act as interpreters for their non-English speaking relatives. How on earth did they manage it?
"I could speak English to my daughter all day every day (ie no financial investment) and if she never saw a book, a DVD, a film or had a trip to the UK, her English would never get very good" - why was bilingualism so prevalent in Europe before the invention of the printing press? I suppose languages were easier back in the old days. Fewer words to learn ...