Few questions.

Don't worry about pokerus it will go away and you don't get the secret key in platinu during the game so just use the action replay

I've gotten it a couple of times. As long as you have the Pokemon infected with Pokerus in your team it will go away after 2-3 days. It actually gives you a higher stat raise at each level-up!

You can get the Secret Key through Nintendo Event but those already finished. Only other alternative is using an Action Replay.

1) Nope. Nothing for me.
2) Yes you can. Here`s how:
2.1) Go to the third floor of Jubilife TV in Jubilife City and talk to the TV producer. Answer his questions "Everyone Happy" and then "Wi-fi Connection" and then save the game and kill the power. Turn it on and go to the menu and don`t start the game because there will be a new option. It is "Mystery Gift". Use it. Choose "Recieve Mystery Gift" and then "via Wi-fi Connection". In about 1 min. you should recieve the Mystery Gift. Inside it is the Secret Key. You need Wi-fi Connection to do it though.

1. Actually Pokerus is beneficial to your Pokemon because it will give them a higher stat raise! But yes it will go away.

2. Yes and you need Wi-Fi, Access to Jubilife TV, and access to the main menu and the game and access to the game. You go to the TV Producer on the third floor of Jubilife City and answer his questions "Everyone Happy" and "Wi-Fi Connection". Then save the game and turn it off. Turn it back on. You will see a different option on the menu not on your saved file but on the main menu and it will be "Mystery Gift". Click on it. Then go to "Recieve Mystery Gift" and "via Wi-Fi Connection" and you will have the Secret Key.