It's not at that point yet. PRC does have a garrison inside HK but they're sitting still. It's possible they could go in plain clothed to collect info and provocateur to discredit the protesters. It'd cause a pretty big meltdown if they were busted doing that.

Aside from that: HK has control over its statutes. https://www.elegislation.gov.hk/index/en/subject

Basic law (the part PRC plays a role) is the constitution. For an overview: https://www.basiclaw.gov.hk/en/facts/index.html

Aside, Here's a better article: http://www.thestandard.com.hk/section-news.php?id=210397

No. The central government maintains a tight grip on the Hong Kong Government, and of course occasionally acts of its own accord in Hong Kong (see the case of the booksellers' rendition), but for the most part they still have to let the Hong Kong legal system operate as it does.

Even in light of these protests, it continues to seem fairly unlikely that Beijing will want to intervene militarily or impose direct controls over the HK government unless things heat up considerably more than they have already.