To answer these question we need to apply the 5th ed Muto Animal baseline of "MuAn/Co 10 which transforms an animal into a human" (Ars p. 118) as it explicitly states that the animal's mind is not transformed.
So firstly the suggestion the introduction to the questions that an animal might gain some human aspects in incorrect; the animal retains it's animal mind and nature. This means the animal will still use Cunning instead of Intelligence, and would still have access to all of its Abilities and suitable Ferocity. This is because the MuAn/Co 10 base is purely a physical change. Any mental change would need a Mentem requisite and discussion of what base that level is (probably by comparing to the MuMe/An 15 baseline that changes the mind of a human into the mind of a beast).
This also means that the animal is still susceptible to Animal spells, and now is also susceptible to Corpus spells, in the same way that a transformed magus is susceptible to Animal and Corpus while transformed by hermetic magic.
Q1. How would such an animal behave once transformed into a human?
Please account for pre-transformation states of mind of the animal
such as calmness, fear, aggressiveness and estrus/rut (in sexual
heat). How much of a mind would it have?
A1. As it has only an animal mind, the animal would try to behave as it did before the transformation. It is reasonable to expect the animal could function "normally" as a humanoid once it has had time to adjust to the new humanoid shape.
The animal would probably be confused or hostile, depending on its traits and its nature. Being "on-heat" is up to you, it could be argues that "on-heat" is part of the beast's nature, or could be argued that it is an attribute of it's body. Go with whatever makes the story better for your group.
Q2. If the animal turned human was killed before the expiration of the
spell, would the animal change back immediately or not until the
expiration of the spell? My belief is that the animal remains in human
form regardless of whether it is alive or dead, until the spell
A2. I agree, the animal will stay transformed until the spell ends. The beast could also be affected by multiple transformation spells, and their effect and duration should be tracked independently of each other to determine which is in effect.
e.g. female piglet is transformed into a young human female with duration moon, then transformed into a male human for sun, and then killed. The corpse will say male until the spell expires at dusk and revert to a human female, and then stay as a dead female until the next moon, when it turns back into a dead female pig.
Q3. Would magi (or any other group of people knowledgeable about
magic) consider eating an animal in human form to be cannibalism or
merely a fetishistic activity that normal people would probably not
wish to do? Would this be a way for a magus with cannibalistic desires
to indulge in them in a relatively safe manner? As in "He wasn't
actually eating people, they were just transformed animals"?
A3. As magi generally like to consider themselves civilized, I believe that eating a human for any reason would be at the very least distasteful, at worst totally taboo/evil. There are (probably) notable exceptions to this in the books somewhere, however given that Ars sets the stories in Mythic Europe/Christian lands, then cannibalism is not acceptable in any form.
It is probably not a hermetic crime, but would be exceedingly off-putting. Suggest that cannibalism is considered far worse than necromancy by Magi.