I suggest that "lay flat" be replaced by "flat bed" in the seating description since "lay flat" often suggests the angled version AF uses.
I finally figured out the second ATL/DTW frequency are complementary. Thus, if there are two flights from ATL on a given day, there's only one from DTW that day. There are 11 weekly departures from ATL and 10 weekly from DTW.
Getting into Central London from LHR is easy, and many of us would encourage it if you have six or more hours on an MR. To maximize time in Central London, get the
Heathrow Express for £32 return. It runs every 15 minutes and takes 15 minutes from Heathrow Central (T1/T3), so it will take a bit longer from T4 as you'll have to take the connecting train.
Heathrow Connect is £16.20 and takes 23 minutes from T4 to Paddington. It also runs every 15 minutes and makes four stops between LHR and Paddington. The Underground is £5 cash each way (£2.70 off-peak on Oyster), but takes about an hour into Central London. It does drop you at a better position perhaps, as Heathrow Express takes you to Paddington, whereas you could get off the Tube lots of places. If staying in London, note that changing trains on the Underground usually requires steps and long walks. This can be unpleasant with lots of luggage. If you've got questions about specific transfers, feel free to ask them in this thread or over in the UK and Ireland forum. (We've got some real Tube geeks over there, and I mean that in the best way.)