R Moshe Feinstein (Even HaEzer IV:92:2) is against raising veal in inhumane ways. He doesn't say the meat is not kosher (see e.g., this answer on MY). Nevertheless, some (e.g., R Moshe Dovid Tendler, the son-in-law of RMF) do not eat veal for ethical reasons.
The questions you raise are nevertheless important and I have also become increasingly sensitive to the treatment of animals in industrial settings and how this impact a Jew's ability to eat meat.
Heshy and Malky Zelcer wrote an excellent summary of the issues in Hakirah (vol. 24 Spring 2018, pp. 173ff, see here but also this important response and its rejoinder).
They bring multiple opinions showing inhumane treatment of animals is prohibited because of tzar baalei hayim
- R Moshe Feinstein writes it is permissible to cause to pain to an animal when done for the fulfilment of a human need (which includes slaughtering them) but not if it is only for economic profit or to beautify the meat (e.g., keeping veal meat white)
- Sefer Hasidim prevents causing un-necessary pain to animals (p. 425, sec. 666)
- R Samson Raphael Hirsch writes one may hurt animals only for sensible human purposes and then only in the least painful manner (Horeb, p. 292-3, 60:416)
- Sefer Hakhinukh writes the animal must not be unduly pained. The Torah allows man to eat from them and use them for all his needs, but not to cause them needless pain
However even if it is not proper to raise animals by making them suffer, their meat is nevertheless kosher and one is not forbidden to eat it. The authors bring a tshuva from R Shmuel Kamenetsky who writes
- concerning actual practice (of raising animals in industrial settings), it is not proper for the descendants of Abraham to do so, whereas according to Igrot Moshe (above) there is even a possibility of an actual violation
- but as for buying eggs and the flesh of chickens that were raised in this manner described above, it appears from the Igrot Moshe that there is no possibility of a violation, for there is no reinforcement of cruel habits, and the purchaser has no part in the tzar baalei hayim, nor is there any fear of being an accessory to the tzar baalei hayim
That answers your third and fourth questions. Regarding your question of how kosher authorities consider this issue, R Menachem Genack, CEO and Rabbinic Administrator of the OU (the largest kosher supervision agency in the world) was interviewed in Yated and said
We don’t believe that it is the responsibility of the OU or
any kashrus agency to be the arbiters of those [worker and animal treatment] issues, because they
are all covered by different federal and state laws and regulations.
State and Federal agencies – the USDA, OSHA, FDA, and the EPA, which
handles environmental issues – have the authority and expertise to
handle these issues. We don’t.
Even though, the OU recently decided to remove its certification from any meat processing plants that use a method of inverting beef cows before schechita which is considered particularly stressful to animals.
In reality, the solution to the ethical issues of animal treatment is small-scale (or family) farming where animals are left to grow on their own and not injected with antibiotics. However it is hard to imagine that this would scale to the quantities required by the market and would likely increase prices significantly.
Some smaller-scale companies have developed "humane treatment kosher meat", e.g., Grow & Behold and KOL Foods. In Israel there is also a hekhsher (certification) called Hai Bari which guarantees to consumers that their meat and dairy products come from farms which implement the highest animal welfare standards. See also here for evolving standards on raising veal that would enable R Moshe Dovid Tendler to remove his ban on eating veal.
If you are interested in these issues, I would recommend this episode of the Headlines podcast which discusses Kashrus and Animal Cruelty — Kashrus of Foie Gras and Veal and includes interviews with R Moshe Elefant (Chief Rabbinic Coordinator and COO of the OU) as well as producers of kosher ethical food.