Upaya in Theravada Buddhism and the Pali Canon

Upaya can be found in the 1st Turning, that is, the Nikayas of Theravada or Agamas of Mahayana, roughly.

Examples (please if someone knows and refers the Pali Canon):

1. Buddha told the woman who's only son died to beg for some mustard seeds from a family who never had anyone died to make medicine to feed her dead son so he could become alive. (Didn't the Buddha know it's futile and absurd? But he told her to do so.)

2. Rahula when first admitted in the Samgha, taking privilege of he the Buddha's son, always spoke misleadingly to make fun of others. In order to teach him, one day the Buddha told him to wash his feet with a wooden bowl. After done, the Buddha told him to drink the water in the bowl, Rahula gaped. Then Buddha's teaching stroke in, telling him speaking misleadingly was unwholesome like asking someone to drink that water... [in Madhyamāgama]

However, you said, "...would the Buddha even tell completely opposite things to different audiences?". I wonder how that related to Upaya?

Mahayana is a big school, so is Zen a generalized name (despite the word rented from Japanese people always just assumed it covers the entire Ch'an tradition). Like a school, anyone can easily put on the school uniform pretending the student. You said, "...certain Mahayana traditions, the teacher may even break the precepts (for example, drinking alcohol), or shout at or hit the disciples, as part of the skillful means (upaya) to teach them." Upaya comes from wisdom, however, since it may go as far as testing many rules limitations, sometimes one (likely teacher) lack of wisdom or not really enlightened can being mistaken or mistaking (or the worst, pretending) himself wise and enlightened, thus abusing the "breaking rules". It requires the student with a sound mind of discernment to choose and protect himself from being abused or misled. Blindly worshiping a person is very dangerous, therefore the Buddha not only in one place spoke of "abiding to the Dharma, not the person" (依法不依人), especially in the day when he was about to enter Nirvana, he repeatedly giving this teaching as the final words.

As far as my study (of the Chinese Mahayana lineage) covers, there's no record of any teacher drinking alcohol, except the monk Ji Gong. But he wrote a poem on it, "alcohol and meat just passed the stomach, Buddha in the heart sits", with the 2nd part always being disregarded but he warned, "if others imitated, same with Mara are they!" which said this wrong he scolded. Ji Gong didn't die of alcoholism. Ji Gong living at the time when Bodhidharma went to China, both demonstrated supernatural powers in many records that they were at least Arhats. Again, you quoted, ...a monk realized enlightenment when his teacher slammed a door on his leg and broke it... Is this story in the Japanese Zen Koan? In my study, I only recalled one bloody incident, that was Huike chopped off his own arm in front of Bodhidharma. It was definitely not instructed by Bodhidharma; likely Bodhidharma was shocked too, honestly. But Huike's determination was moving.

Upaya with the nature not appealing to any established rules and formats, not only it's often abused in later days, also the term corrupted and hijacked. Upaya founded upon wisdom, but often wisdom is missing in the practice just used it to excuse selfish or unwholesome deeds.

AFAIK, breaking precepts that result in unwholesome Karma is never excused in Theravada Buddhism. Especially the five precepts.

Here are few examples of the Buddha using Upaya:

• Showing nymphs to prince Nanada to break his attachment towards Janapada Kalyani.
• Breaking Khema's infatuation with beauty by showing a beautiful maiden deteriorate in front of her eyes.
• Teaching the stanzas to old brahmin who was mistreated by his four sons.

There might be teachings that appear complete opposite if you take the literal meaning of the words or disregard the context. But the teachings never conflict when you understand the full picture.

The Buddha would scold a monk to get him disciplined, but he would never use physical force. Such a need wouldn't even arise to the Buddha. Although monks are allowed by Vinaya to strike another in self defence.

The Buddha may decide not to keep to a lesser(non-offensive) Vinaya rule on certain occasions, but the monks do not have this freedom.