1. Regarding Receipt Validation
Receipt validation is your means of performing extra security checks before you unlock the users content. Think of a receipt as a lotto ticket. A client comes to you with the "Winning Ticket". Depending on how valuable you consider the reward to your Lottery you can either give said individual their winnings, or you can contact Apple and check whether the user actually purchased this ticket through them.
The client could have got this lotto ticket through some malicious means be it a jail broken device etc and so its really up to you to decide whether you want to verify the receipt or not.
2. Regarding Renewal Notification
The official documentation states the following:
After a subscription is successfully renewed, Store Kit adds a
transaction for the renewal to the transaction queue. Your app checks
the transaction queue on launch and handles the renewal the same way
as any other transaction. Note that if your app is already running
when the subscription renews, the transaction observer is not called;
your app finds out about the renewal the next time it’s launched.
So you should definitely be getting receipt renewal notifications in your app. Don't forget that you should always add yourself as the observer of StoreKit's paymentQueue in order for this to work, something like this in your app delegates didFinishLaunching would work:
//where YourObserver is an object that is alive for the apps lifecycle
[[SKPaymentQueue defaultQueue] addTransactionObserver:YourObserver];