Obviously this was very upsetting, inconvenient and you are very angry.
It appears from your long retelling that you did indeed make a change to your booking. And that despite it appearing to be confirmed and there being no additional payment required, it was subsequently cancelled.
Suggestions that they bumped you off so they could make more money and have been gaslighting you since may feel right through the red mist. But it is also possible that there was some processing error in the back office or at their payment provider which placed an error code against the transaction, and your ticket was actually subsequently autocancelled rather than you being picked on. There have been many reports here of payment processing issues recently (you may have missed them if you haven't been around). Whether the source of those be at QR or their payment channel providers is never clear.
I suspect it is very highly unlikely that you have been specifically targeted, either by revenue management or the payments company. The problem here is that there probably was a processing issue somewhere when you changed your booking, your ticket was then autocancelled, but that you had not been informed by the time you noticed it yourself.
The first key thing is to get rebooked and get home. You can whinge about the extra taxes but those are outwith the control of the airline. If you have suggested flying from a different jurisdiction with a different tax regime, I would suggest that might be a 50:50 issue, but something to discuss with them later.
EC261 does not apply to this journey, but that is no reason not to write to customer services. Explain calmly, chronologically and clearly what happened. Include whatever evidence you have of what changed when, plus receipts for any additional costs incurred. Use those costs plus a calm description of what EC261 might include for a similar incident as a starting point for your discussion (they will tell you EC261 does not apply but it's a good reference point for any gesture they may decide to make). And then send it via the complaints drop down here:
https://www.qatarairways.com/en-th/h...ntact-us.html?
Ranting at them and throwing accusations around will not achieve anything. So I would gently suggest adopting a somewhat calmer, much more concise and less emotional tone than your post above.
Good luck