It's almost instant. You can find the account information in your mailbox and log in account right after your payment.
Sure, the account will be yours forever and you can do anything with account after your purchase. We are the biggest and most trusted MMR Accounts seller in all servers. You won't find any seller which have more accounts than us.
After last several updates, some of the users (especially users with multiple accounts or lower than 8000 behavior points) has been put into special queue by Valve system when they use new account that didn't login to their PC/IP address before. This based on the factor that the new account didn't have activities record on your PC/IP address yet. To fix this, you simply just need to play unranked matches constantly for 1 week, after that you can find ranked matches normally cause the account now already had your activities record on the account.