Flight cost from A->B/B->A is $2k more than C->A->B/B->A->C. What are the dos and don'ts

Lets say I live in A(USA) and need travel to B(Asia). The cost of a ticket from A->B is $2000 more than a ticket from C(USA) -> A(USA) -> B(Asia).

C and A are 90 minutes apart and there are multiple flights daily. Can I or can I not do the following:

Book C->A->B and turn up at A and tell the checkin staff that I came there through alternate means. I will obviously have to be before the flight from C departs to avoid being a no-show.
On the way back from B -> A -> C, can I disembark at A after requesting the checkin staff at B to book the baggage only until A?

I've already booked the C->A->B/B->A->C flight but I want to see if I can save myself a little hassle along with a lot of money.

Thanks

Best Answer

To add to a detailed answer of @jcaron, there are two more potential issues:

If you have a flight C->B->A and intend to leave on B, it is possible that your C->B flight gets canceled and the airline would reroute you to C->D->A or even C->A flight (they have all the rights to do this since your destination is A). However even in such cases the airline tends to be flexible, so if you insist on flying exactly C->B they would still accommodate you - but it might not be the same day or even next day, and you won't be compensated then for the delay.
If your C->B leg is Asia - USA, and you have checked in the luggage (expecting to pick it up in the US customs and leave) and your luggage is lost, by default it would be routed to A, and not to B. In this case however you can request the airline to route it to B, and even explain that your trip to A without luggage makes no sense.

And three more notes:

Some countries give you more flexibility. For example in Italy you can skip some segments from your itinerary without penalties. See for example item 3.3.8 of the conditions of carriage for Alitalia: https://www.alitalia.com/en_us/booking/general-conditions-of-carriage.html

"However, only in case of tickets sold in Italy, if the first leg of the flight (or a segment of it) is not used for any reason, the request to hold the validity of the Ticket for the return flight, can be accepted only if previously communicated to Alitalia’s Contact Center..."
Some airlines position themselves as "point to point" and thus don't care at all about missing segments. Ryanair, for example. However you're not going to save any money with them by booking hidden city, because they charge per segment.
The legality of hidden city ticketing has been tested in Berlin court, and Lufthansa has lost this case. See for example https://liveandletsfly.com/lufthansa-loses-lawsuit/

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