The Taiwan Strait is about 70 nautical miles wide at its narrowest point. By general convention, a 12 nm territorial water and another 12 nm contiguous zone either side only gives a total of 48 nm meaning that there are still 22 nm of high seas between the mainland and Taiwan if China sees itself bound to the United Nations Convention on the Law of the Sea, to which it is a party. (China can and probably does claim the 200 nm exclusive economic zone around its waters but that does not convey any of the rights I will be talking about below in the treaties.)
Even if China were able to successfully argue that the entire Taiwan Strait be territorial water or contiguous zone, the Convention would disallow China doing any of the four points you mention to merchant ships as this would go against the Right of Innocent Passage. (Except maybe requiring vessels to identify, i.e. show their flag.) The Right of Innocent Passage is codified in article 17 of the aforementioned convention and the meaning of the words passage and innocent passage is explained in the following two articles. The most important in this context is article 19 which reads as follows:
Article 19
Meaning of innocent passage
Passage is innocent so long as it is not prejudicial to the peace, good order or security of the coastal State. Such passage shall take place in conformity with this Convention and with other rules of international law.
Passage of a foreign ship shall be considered to be prejudicial to the peace, good order or security of the coastal State if in the territorial sea it engages in any of the following activities:
- (a) any threat or use of force against the sovereignty, territorial integrity or political independence of the coastal State, or in any other manner in violation of the principles of international law embodied in the Charter of the United Nations;
- (b) any exercise or practice with weapons of any kind;
- (c) any act aimed at collecting information to the prejudice of the defence or security of the coastal State;
- (d) any act of propaganda aimed at affecting the defence or security of the coastal State;
- (e) the launching, landing or taking on board of any aircraft;
- (f) the launching, landing or taking on board of any military device;
- (g) the loading or unloading of any commodity, currency or person contrary to the customs, fiscal, immigration or sanitary laws and regulations of the coastal State;
- (h) any act of wilful and serious pollution contrary to this Convention;
- (i) any fishing activities;
- (j) the carrying out of research or survey activities;
- (k) any act aimed at interfering with any systems of communication or any other facilities or installations of the coastal State;
- (l) any other activity not having a direct bearing on passage.
(formatting slightly adapted from source to fit with Stack Exchange's Markdown)
At best, one might argue with 2(d) (propaganda) but that would be a very serious stretch considering a merchant ship tavelling (e.g.) from Singapore to Busan without any intent of calling in China.
Furthermore, articles 27 and 28 specify the very limited cases in which the coastal state has and may exercise criminal or civil jurisdition of a merchant ship exercising its Right of Innocent Passage. Essentially, the only loophole that remains is if such measures are deemed necessary for the suppression of illicit drugs or psychotropic substances.
Your point (4) is rendered impossible by article 17 (ships of all states enjoy the Right of Innocent Passage) and article 24 (the coastal state shall not [...] discriminate in form or in fact against the ships of any state or against ships carrying cargoes to, from or on behalf of any state).
The above is relevant if China were to declare the strait territorial waters; if China were to declare it as contiguous zone, it has even less rights.
Therefore, any measure as you outlined when exercised against merchant ships would be seen as contrary to the convention and would likely create a strong response from other countries. I don't think that would be a price China is willing to pay to demonstrate its sovereingty claim over Taiwan at this point in time.
