Visa/Mastercard also have a dispute system which can result in a chargeback to the merchant... but this is meant for where the charge is unrecognised/unauthorised and this doesn't appear to be the issue here. Credit card companies seem to be tightening up as cardholders are increasingly abusing the system.

Where is the OP? We don't know....

Beyond the above scheme, the extent to which credit card companies will get involved varies from country-to-country. In the UK, the Consumer Credit Act 1974 gives some protection as (subject to some exceptions) the credit card company is jointly liable for providing the goods/services purchased. For this reason they can "get involved" - not just for ethical reasons but also because ultimately they can be legally responsible. But they will only do this if there is a clear failure - eg goods not delivered. There was a recent court ruling that held this protection applies to UK cardholders even when using their card overseas.

The OP writes "they left me without any assistance in Siem Reap" - is this a contractual obligation or required by local law?

And then "Having already paid for the tickets to RKA, I had to spend another USD 1,000 for other tickets to have a chance to get back home"

Really? Did they say they were never going to take the OP home? Often when airlines have a problem it can involve a long wait for another flight. Even a confirmed reservation is not a guarantee that you will be on a particular plane. Some countries have a scheme for guaranteed compensation if you are denied booking and many airlines would indeed offer assistance (hotels etc). Often they will accomodate you on the next available flight or reroute - but that doesn't mean they have to put you on the quickest alternative regardless of cost.

"RKA rejected my request to compensate me for that amount. This is already totally unfair" .... I don't think so - especially if the cost of the indirect flights taken by the OP on other airlines is out of all proportion to the fare paid. It was the OP's election to do this.

Unless RKA said that they couldn't/wouldn't carry the OP on a later flight then I think the OP has a weak case. We need more information about what they really said, and what can be supported by evidence so long after the event.

If the OP has a record of their promise to refund the unused portion of the ticket (eg in writing or contact details) then I would use this as a basis of a complaint which should also include all the dates and times - and what was actually said and by whom in what capacity. Vague references to their "shameful and dishonest methods" won't help.

Otherwise this is an insurance matter - if you can't bear the risk of an event occuring where you cannot hold another party legally responsible.

(Don't want to sound harsh or unsympathetic... but too many unknowns here.)