The XP boosts are a passive increase so long as you are a member, and are always on. It's not a consumable or anything like the Blessing of Asura.
Free accounts currently get 2 slots, and then +6 for Membership (for a total of 8). New accounts made after the change start with 1 slot and get +7 for Membership, for a total of 8. I have to check with the account admin tomorrow at the office about how exactly that'll work for existing accounts, just to make certain of details before I start saying one thing or another.
Offline Levels being removed is something we know not everybody is going to think is awesome (as is the case with any feature that is removed), but I think it'll be better this way in the long run.
If you aren't a Member now, you can't access Dragon's Spine, nor RotGS zones beyond Gateway to Khitai. If you are not a Member when the changes go live, you may purchase the DS pack for some amount of Funcom Points (price TBA) and then you'll have access to those playfields + raids forever, regardless of your Membership status. If you're a Member when the changes go live, you get Dragon's Spine for free, and then if you decide Membership isn't for you, you may let your Membership lapse but still access the zones. I hope that makes sense.