If I miss my firm and go to my insurance, does that mean it's likely I won't be in Halls?
For some unis, you will 'buddy up' with someone else in the same room as you for around a few days to a month, until there is spare accomodation.
It depends on the uni. If you don't get in straight away they will have a waiting list for when other people drop out.
It very much depends on the uni. Some will guarantee all first years, some only firm choice applicants, some will only be able to accommodate a percentage of firm choice applicants. Some places allow (or, indeed, require) insurance applicants to apply for accommodation in advance of getting their results; others won't allow them to apply until they know they're coming. Availability of accommodation is one factor worth taking into account when you choose your firm and insurance choices - especially if you have a few options you can't decide between. It's generally a good idea to look at what the arrangements are for each uni you're interested in, as they will vary quite a bit.