Hong Kong and China are massively different, but people are REALLY over-exagerating these "problems".
There isn't a "custom" of s**ting in the street; that's just ridiculous. You will, occasionally, see parents letting their kids piss into a flower bed or something, but this idea that people seem to have that people are just popping a squat whenever and wherever they feel like it just not true at all.
Spitting is definitely a thing, and it's gross, but it's not specifically a Chinese thing. This idea that crapping everywhere "happens a lot" is absolute bulls**t. The reason the occasional story makes the news, is because it IS news. Do a search and you'll find lots of examples, but most of them will be from Hong Kong or Taiwanese news sources, who LOVE to paint themselves as better than the mainlanders and complain about they're "ruining" Hong Kong. The reactions from most Chinese people to those stories are equally/more disgusted than everyone else.
The park will be fine. The majority of HKDL's guests are from mainland China, and no, the cleaners aren't having to walk around with pooper scoopers.
Parks in China are also fine. They have toilets in them and everything. I've been to one or two now to know.
Yeah, the Chinese can be loud, obnoxious and EXTREMELY selfish. They'll happily put a camera right in front of your face during a show, for example, or push their way to the front to get a better view of something. They'll ignore signs for the most part. Having said that, actual queueing is less and less of a problem these days, especially in places like theme parks.
It will be crowded and stressful. The park won't be getting pulled to pieces, and you won't be stepping over piles of s**t.