Here's the answers to your questions
1. Yes - an iPhone 4 with the most update software
2. About a week
There were just sent as iMessages - my old nokia got things people sent as texts. I don't think people got messages saying they hadn't been sent, which I assume would happen if it failed?
It seems to be a pretty common problem but most people just talk about how to turn off iMessage rather than how to find iMessages that have been lost.
I've had people looking round my flat who might've sent them so need to get them back if possible.