I have done this a lot over the years across multiple shards and this is how I personally do it.
I round up all the PEs, acid eles and plague beasts from the red marked area and lure them into the green marked areas, this can take a little prep time (best done with a mage for easy invis) the red area is littered with slime spawn and once clear of the 'annoying eles' it is a breeze to farm the red area back and forth. If the spawn dries up a little I weave a circuit of the pink area in.
Regarding the drops, RnG is what it is. Sometimes I've gone a long time with no key drops and yes it gets quite disheartening but you just have to push through. The keys only spawn in the Palace of Paroxysmus as far as I am aware.
Regarding the Paroxy mount drop rate, the quoted 5% feels about right to me. It's certainly not a super rare drop, it's just the time consuming process to key and get in to Paroxy in the first place that puts a lot of people off farming it.