Is it possible for me to lose my msdn admin account after purchased?
You need to follow a couple of easy rules that are in the How To (included with every MSDN Admin Account), and your account will serve you the whole period which is about 30 months. Don't abuse it and you will be fine.
I see you are selling admin accounts on here. Is there any chance you also sell invite accounts instead, for a lower price since I am only really interested in those?
Sorry, we don't currently sell invites. Purchasing MSDN Admin Account is cheaper then buying 5 invites separately, and gives you the full control of your accounts.