I am wondering now whether it's possible to perform legal actions against my own insurance arguing that the leak may be long term and not sudden, but it's sudden to me when I noticed
My homeowner's insurance policy does not cover mold damage unless it results from a covered loss (basically, either a fire or a big storm that makes a hole in the building envelope that lets the damaging water in).
The unfortunate truth of the matter is that insurance policies are not as good as they could be, and exclude many things that are not the homeowner's fault. Before you try suing your insurance company you should read your policy. I will guess that you don't have coverage for this event.
I am really not good at reading the CC&R.
The only way the CC&Rs can help you is if they say that unit owners are automatically responsible to pay for damage to other units that result from anything that goes wrong in their unit even if it wasn't their fault. Condos with that kind of wording do exist but they are rare.
It's kind BS that I paid north of 10K for something I hardly have any fault in. Any advice will be appreciated!
All I can suggest is you learn what you can from this experience. Always do your best to set some savings aside for unexpected expenses (and in case of job loss, too, of course). Consider saving money on your insurance premiums by increasing the deductible. And make it a habit to pay attention to your home.
Listen for unexplained noises and try to figure them out. Check common sources of trouble such as your air-conditioning system condensate drain. If you have your own water heater make sure it has a harmless path for the water to drain out if it starts to leak, or if you can't do that then at least buy a leak alarm for it and let your neighbors know what that sounds like. If your bathtub or shower has grouted tile on the walls, make sure the grout is in good enough condition not to let shower water into the wall. Keep a plunger handy, and never flush anything down the toilet that should go in the garbage (e.g. tampons or dental floss).
Except possibly for the listening part, none of the above would have saved you from having this experience. But if your neighbor had listened and looked, he probably could have saved you both a lot of repairs.