Use iPhone without Carrier

1. An iPhone can work fine without a carrier, but you won't be able to make Cellular calls without an active service plan from a carrier. The service plan is what lets you among other things make cellular voice calls.

Without a service plan form a carrier, then you are limited to wifi as you said, and the iPhone would be disconnected from everything when not on wifi. No messages, no internet based calls etc...

2. You only need to get the iPhone unlocked if you plan on using it with another carrier. Using it on Wifi does not require it be unlocked from Sprint. If you do wish to have it unlocked, then that is the only thing you need to request.

3. You can make VoIP calls in wifi without a sim card. These calls would be made through 3rd party apps such as Skype. You would need a sim card with active service to make Cellular Calls.

Apps like Whatsapp and Viper, and even Facetime on an iPhone need a functional cell number to work, which would require a active service plan and sim card.

The only way to get a sim card, is to get one from a carrier with service.

Well, you can use your iPhone without a carrier, but only as an iPod Touch. This means that you can use it via Wi-Fi with your applications (like Facebook, WhatsApp, etc.):

1. You can't make (traditional) phone calls without a carrier. You can make VoIP calls (Skype, Viber, WhatsApp calls, Telegram Audio,...) and always via Wi-Fi;
2. I sincerely don't know, we don't have locked phones since 2007/2008 in my country (Italy). I recommend you to ask directly Sprint for the requested documents;
3. As mentioned before, you need a carrier to make phone calls, and to connect to a carrier you need a SIM. Remember that you can use VoIP apps if you have Wi-Fi.

Hope this helped you!