score:2
There are several ways to go about this.
Naturally, I would recommend option #3.
If you miss any leg of a flight, all subsequent legs are canceled automatically; this is a standard rule across all major airlines. Still, given the vagaries of airfare pricing, many people do deliberately book final legs they do not intend to fly, a tactic known as hidden city ticketing. This is technically in violation of your contract with the airline, and you could lose frequent flyer miles or status over it, though it's only enforced in fairly extreme cases.
The larger danger comes from ordinary operations. Your Contract of Carriage (or Conditions of Carriage) with the airline is to get you from A to C and from C to A. They can, at their discretion, change your return flight from C-B-D-A to C-B-A or C-F-A or some such, bypassing D altogether.
As for frequent flyer considerations, you will not receive any miles or segments for flights you do not fly, as a general rule. They base whether or not you fly on whether or not your boarding pass was scanned. In your example, you can still expect to receive miles for A-B, B-C, C-B, and B-D, just not D-A.
To head off a potential followup, no, you cannot have your boarding pass scanned and then leave. The flight attendants do a head count prior to closing the door, and after they determine that a passenger has left the flight, at a minimum your boarding will be canceled, and worse, airport security and/or law enforcement may want to question you.
Upvote:1
Airlines generally don't like this, because it's often used for Hidden City Ticketing, in which you buy a ticket A-B-C intending to only go to B, because it's cheaper than the fare for A-B.
The answers to this question cover the potential issues, but in short:
I'm not sure whether the miles will be credited, although airlines have been known to cancel the frequent flyer accounts of people who do this regularly.