EVE’s market transaction is fairly robust, and has taken a lot of hammering over the decades, so it’s unlikely to be broken in this case. It does however have it’s various oddities and “takes some getting used to” interface design, and that’s likely what you ran into.
The transaction was started and then cancelled due to lack of ISK for fees. So the Plex probably moved within your inventory in the process. It could be in the Plex vault. It could be in Station inventory. It could be in a different station inventory if you happen to have moved stations while dealing with the issue. Heck, it could even be in a ship cargo bay if you were moving other stuff around at the same time.
There is a “Personal Assets” icon with a Search window for locating assets. I’m not in-game atm but think it’s Alt-T for keyboard shortcut. Or look in the EVE menu. That should help you locate it.
As for tickets, it’s not wise to accept what you see posted on the forums without reservation. People often exaggerate delays, or never actually posted their ticket in the first place due to an error, or they posted some half-baked midnight rant that the GMs can’t even figure out. In fact probably 80% of the “this game sucks, support sucks, game is bugged” reports here are user error due to some interface snag the poster wasn’t familiar with.
That partly explains some of the dismissive replies you’ve received. Another part is that you get better answers if you start from a “Hello, I did X in-game and got Y result which seems broken to me, what should I do next?” - rather than jumping straight to “Game is broke and this sucks and CCP gets no more of my money!”. Longer-term posters have seen this before and it often turns out to be an user issue.
(I note you were somewhat dismissive yourself of other posters complaining about various aspects of EVE in other threads.)
Opening a ticket is helpful, it reminds you at the time to get screenshots, to check your transaction logs, and gives you a record of when and where and what happened while it’s still fresh. It’s true, sometimes support replies in short order, and sometimes they take two weeks. And sometimes you have to explain the situation twice more after they do reply because they don’t understand the problem (they’re not players, generally). But it’s better to have filed it than not, and once they do understand the issue they’re generally fairly helpful.