Not sure what secret codes your talking about? You only need to run a 2nd cable if they are going from Sky to Sky+HD. If you already have Sky+ then you are fine. I bought my HD box of ebay and just plugged it in, phoned up Sky to get HD enabled and its been fine ever since.
Just today I also cabled up a 2nd HD box myself by just running a cable from the LNB.
Correct, but I believe that a virgin HD box is a slightly different install and is not plug and play but I stand to be corrected if that is not the case
You are corrected - see below
I dumped Sky+ last year - issues with crashing and got one of the Humax recoding boxes in December that does all the same as Sky+, including pausing live TV etc and with HD. Simply plugged into the old Sky+ aerial (which has the double lead facility and you don't have to change the aerial orientation) and with freesat and freeview I get all the channels I want. The box cost £300 outright - no more £40 monthly payments to Sky so it has just about paid for itself already