T5.
Delta uses Heathrow terminal T4.
T3.
There are various places you can fly to from Heathrow Terminal 3 such as Chicago USA, Amman, Oslo and Vancouver Canada. It is an International terminal that help you in your travel destination.
Yes
Numerous airlines fly from Heathrow to Austrailia, from different terminals.
Delta fly in and out of Terminal 4 at London Heathrow. See related link for more information.
Terminal 3
9 hours 45 minutes NON Stop on British Airways from Phoenix to London Heathrow Airport
Most would fly into either London Heathrow or Gatwick Airport.
That info would come from the airline.
This depends as London has 5 airports servicing it. Generally speaking, although probably the furthest away, Luton Airport is the cheapest to fly into and Heathrow would be the most expensive.
No, ryanair has never flown from Heathrow. Their main London base is Stansted; but they also fly from London Gatwick and London Luton.