i am not 100% sure on this answer but i saw no one else has replied so i thought i would try to have a go.
If you join now u will receive the game so if u pay that and then unsub once the code is received you wont be forced to pay next month but if you decide u want next months instant game then you pay again you will receive it yes because that is what you payed for and if u buy it earlier on then you will be able to receive the rest of the game for taht month as well (for monthly).
again i would count on my words 100% but this is what i can infer from other peoples info .