Exclusive just means only if you're a member. But you are, you just signed up.

I think they term it that way for 2 reasons: first, to try and build some fake loyalty and familiarity (you're already exclusive!) but secondly it's so their normal rate is the same as other booking agents - they can offer it cheaper by making it "member exclusive"

With regards to flexible / cancellable that's usually exactly what flexible means - you can cancel it (and as a flow on, can change the dates, since worst case you can just cancel and re book)

Extra guest charge is most common with a rate that includes breakfast and lastly I've always assumed those double points rates are a cheap way to abuse people booking business expenses.

Hope that helps...