I thought the same about Salford, but the road that you can see in the background on the bridge is a major motorway for access to Manchester and it's also pretty close to the Trafford Centre, which is one of the biggest and most well-known shopping malls in the country, so I don't think there will be too much of a problem. Also I think the area surrounding the stadium is going to be built up in the coming years, but don't quote me on that.
Warrington are only filling in two of the corners, which will take the capacity to 15,500. The other two will take it over 18,000, but that isn't neccessary just yet. IMO the stadium will look far better with the corners filled in.
Salford city council are paying for the development (being built by Peel - like seemingly everything in the North West) and a surrounding retail development. The income from the latter will pay for the cost of the former.
Are Warrington including more corporate areas in the expansion? IIRC they are completely sold out, and some on long-term deals, at the moment.