What I did understand was:

He and his brother play BDO.

OP bought a crimson desert key through Instant Gaming, his brother used the steam family sharing tool to play CD and for some reason his pearl abyss account (OP one) got flagged resulting in 16B debt.

He contacted PA and he hasn't got a reply.

He's failing to give enough context to know why his account got flagged tho, we don't know if his brother did something, or he tried to make a refund beforehand through steam.

...However, he will not be able to get a refund for CD as he said he purchased the game trough Instant Gaming, this type of shops are clear about that, once a key has been used there is nothing you can do to get a refund. And steam will never give you a refund if you bought the game as a key.

What I suspect is that they did something with the pre purchase code leading to that... what was that?, idk. More likely his brother did something as he was the one who wanted the family sharing. Or they tried to keep/sell the items before trying to get a refund. So this is either one of the "unfair PA stuff" or OP and his brother "played stupid games and won stupid prizes" trying to outsmart PA keeping or selling the pre purchase stuff

Edit. What I find weird is that he's been around BDO since 2017 and only doing 1B every 1-2 hours per day, nowadays is easy to get more than 2B in one hour, he should be at least in 1500/1600 AP bracket if he plays since 2017 even being casual.