U.S. Airways is part of the Star Alliance network, while Delta is part of the SkyTeam, so no, they're not flight partners.
Typically, you can claim miles for your frequent flyer account up to 12 months after the flight.
Delta offers frequent flyer miles that they call Delta SkyMiles. You can find out the benefits, rules, and answers to FAQ's at http://www.delta.com/skymiles/index.jsp
No they are not. They merged with Delta quite a few years ago.
You can apply for most frequent flyer programs on the airline's web site. Be sure to present your frequent flyer card when you check in for each flight, because sometimes it can be a hassle to get credit for miles retroactively.
Usually 25,000 for a round trip ticket
Any airline in the Skyteam alliance - but particularly with AirFrance and KLM
Your best option would be to visit the Delta site. At http://dn.delta.com/skymiles/purchasemiles/buy/terms they break everything down for you and let you know the terms of purchasing flyer miles through Delta.
There are a number of frequent flyer programs available from all major airlines. I personally like the Hawaiian Air Miles program as well as the Delta Sky Mile program.
American Airlines Advantage, Delta Sky Miles, and US Airways Dividend Miles are rated as very good programs by Business Travellogue. The tell you what the rewards are and how to get them at http://www.businesstravellogue.com/best-frequent-flyer-programs.
After your flight, you can earn frequent flyer miles by signing up for the airline's loyalty program, using a travel rewards credit card, booking flights with partner airlines, staying at partner hotels, and participating in promotions or special offers.
Reviews covering Delta Airlines' frequent fly program, known as SkyMiles, can be found on websites such as The Points Guy, About, and The Frequent Flyer Authority. Generally, reviews of the program are negative, citing a poor rate of miles earned compared to other companies. Delta revamped the program in 2013 in an attempt to make it better.
The miles usually need to be claimed within 6 months of travel