Is it okay to board only the second flight of a return ticket? The return ticket is much cheaper than buying one way

In general, if you find a way to pay less, the airline won’t let you. In particular, if a return cheaper than a single you can bet then the airline will prevent you from using a return as a single if it possibly can.

This depends on the ticket rules. Usually, the rules depend on the airline. Usually, “traditional” airlines sell there-and-back tickets, where a return costs a lot less than two singles (sometimes less than one single), and you must use all the segments in sequence: if you don’t take the first flight, then the airline will cancel the ticket (examples: British Airways, Singapore Airlines). Usually, low-cost airlines sell independent tickets, where a return is just two singles (and priced as such), and in that case you can use the return if you missed the first flight (examples: Easyjet, Ryanair, Wizzair). Note usually — there are plenty of exceptions, you need to check on a case-by-case basis.

Airlines set prices based on yield management, not on cost. If there’s an apparent way to bypass yield management, then usually the airline is wise to it and has made a rule to forbid it. For example, if an airline has noticed that only a few rich travelers do complex travel involving one-way flights, then they’ll make single tickets very expensive. If they do that, you can bet that they’ll try forbid anything that allows people to book a return ticket and only use half of it. Technically, using the first part of the ticket but not the second one may be a breach of contract — but the most the airline can do if you miss the second part is to cancel the ticket which you’re no longer using anyway. But if you try to use the second part without the first, the airline will cancel the ticket before you use the second part. Airlines also sometimes decide that people from A are richer than people from B and so sell A–B–A for more than B–A–B. Again they arrange to prevent people from A from using the cheaper fare. (Sometimes if you fly the route multiple times you can get one A–B–A ticket for the first flight and for the last return, and B–A–B tickets for intermediate flights, but this is often difficult because you need to know the dates long in advance.) The same kind of reasoning explains why a ticket with a changeover A–B–C is sometimes cheaper than A—B (people from C are poorer, or there’s more competition for A–C than for A–B) — and again the airline won’t let you take the return flights if you haven’t gone all the way to C.

Low-cost airlines often do their yield management differently, on a strictly per-flight basis. If the airline prices A–B–A as A–B plus B–A then it has no incentive to tie the second flight to the first.

In summary, if there’s a “nonstandard” way to get a cheaper price, you can bet the airline has thought of it and has arranged to forbid it.

No your idea won’t work this way. Practically all airlines will have some kind of disclaimer in their terms and conditions that state that you must use the legs of the journey in their intended order. For example Singapore Airlines:

18. COUPON SEQUENCE
Unless prohibited by local legislation, flight coupons are valid only if used in sequence. We are entitled at our sole option to refuse carriage or recompute the fare if your flight coupons are not used in sequence.

And more extensively:

Para. 3. Coupon Sequence.

(a) Carrier will honour flight coupons, or in the case of an electronic ticket, an electronic coupon, only in sequence from the place of departure as shown on the ticket.

(b) The ticket may not be valid and Carrier may not honour the passenger’s ticket if the first flight coupon, or in the case of an electronic ticket, an electronic coupon, for international travel has not been used and the passenger commences his journey at any stopover or agreed stopping place.

(c) Each flight coupon, or in the case of an electronic ticket, an electronic coupon, will be accepted for carriage in the class of service specified therein on the date and flight for which accommodation has been reserved. When flight coupons, or in the case of an electronic ticket, an electronic coupon, are issued without a reservation being specified thereon, space will be reserved on application subject to the conditions for the relevant fare and the availability of space on the flight applied for.

(d) Where applicable, the passenger coupon and all unused flight coupons not previously surrendered to Carrier shall be retained by the passenger throughout his journey and shall be produced and the applicable flight coupons surrendered to Carrier at Carrier’s request.

(e) If the passenger fails or has failed to use the flight coupons, or in the case of an electronic ticket, the electronic coupon, in sequence, Carrier is entitled to recompute the fares in accordance with Carrier’s Regulations for the use of said coupons and the passenger is liable to pay to Carrier any difference between the recomputed fares and the fares already paid by or due from the passenger.

If I continued searching, I would find something along these lines int the terms and conditions of all major carriers.

It is well-known that one-way tickets are much more expensive than return tickets. The idea is that people booking one-way will probably be somewhat desperate or sitting on a bag of money or whatever; in any case, they obviously sell so airlines have no reason to rejudge.

Instead, the typically applied countermeasure by price-sensitive customers is to buy a return ticket in such a way that the return flight is forfeited. In your case, you want to go SIN–SFO. You should search for return tickets SIN–SFO–SIN. These should still be cheaper than single fares SIN–SFO. You still forfeit one leg, but that will just count as ‘no-show’ and typically have no negative consequences (because it’s the return one).

First off, no it is not safe to discard the first leg. If you buy a return (or any multi leg) ticket, missing the first flight on it (or any interim flights if there are more than one) will void the remaining flight(s) on the ticket.

As for why the one-way ticket is more expensive, that has already been answered in detail here: One-way versus return airfare tickets