2) This is a follow-on to their last round of funding, not a separate capital raise. The valuation did not change. They likely had an interested party that missed the boat, got nervous about being left out, and agree to the same terms. Little distraction for the company + money at known terms + small stake (.4%) + press = #yes.
3) AirBnB takes a 15% cut on bookings. A stable company might have a price/earnings ratio of 5x. So for them to have a $25 billion valuation, that's $5 billion in revenue or $33 billion in bookings. That means they would need to grow ~24.5x their current size (either without raising again or raising at the same valuation) to justify this.
According to an industry association[1], sales in 2013 for all lodging in the US was $163 billion. Yes international, but that's for the largest GDP in the world and there are non-zero costs in marketing/sales/regulation/data security to entering other markets.
[1]: https://www.ahla.com/content.aspx?id=36332
EDIT - I was wrong about the fee, thank you richiezc!