DALI Protocol: Is it valid to include this for manchester decoding convenience?
There's no need for a preamble, this is a very simple protocol. There is a minimum time between frames which means you can very easily detect when the line is idle. Then, once the line goes low during idle time, you measure to make sure the line is high at 3/4T and you know you've had a start bit. 

It's asynchronous because there is only a data line, not clock and data. That's nothing to do with DALI per se, that's just a standard communications term. 

If you look at the Atmel apps note you referenced, it talks about two different ways of doing the decoding. Ensure your sampling time is quick enough for your chosen sample based method. Personally, I think the mixture of edge interrupts and timer interrupts is less of an overhead as you're not having so many timer interrupts to ensure your sampling time is fast enough. Which leads nicely on to...

..."how the standard does it". It doesn't specify how it is implemented, just that it must be done in a robust way to ensure it works under all circumstances.