lohpit,
"if I paid for P1 I don't consider it "beta".".
I didn't say P1 was in beta, I said the Preview is not in beta yet - and meant MSFT should be discouraging people from adding domains and so on to a Preview service. They're already aware of how many problems the poorly thought out domain process has caused in production P and E accounts, so allowing domain additions to a Preview product that's only intended for giving an idea of how the new product will look is foolhardy.
In general, though I'm sure well intentioned, this Preview release has added another layer of confusion to what is already an ill defined and highly unintuitive service.
As for domains, deleting ALL users is what has worked for me to remove a domain. I've also read domains can get tied up in FOPE which requires a call to support to get released. MSFT should be giving you the contact details. Or if you search the forums you should find several threads about it.