Yeah, those darn instructions get in the way of having fun everytime. (for me it would be P2, right off the compressor.)
Always add any powder into the liquid, when I mix my investment upto 20 kilo at a time I use 90% of the recommended water and then add additional water to achieve the mix I want. Its probably a bit wetter than they specify but after mixing and degassing it's started to thicken slightly when I pour.
Probably ash. You can get a low ash PLA filament especially for lost PLA casting https://polymaker.com/product/polycast/