Human Familiars Possible?

True, but it's possible make one Familiar Bond-breaker Ritual.

Doesn't help if the penalty is death of the familiar, not losing the familiar.

Yes. Maybe, the Break of the Bond and one Perdo Mentem than left the person like one empty husk, or to be jailed long ago for all the time. Or maybe, they could rule than one hooded or masked Hoplite kill like one executioner or hangman. This is Mythic Europe, the Death Penalty is not un-heard, and Wizard March is likely one death penalty.

However the death of an innocent to punish the guilty is virtually unheard of outside of the nobility (where the child may well be in residence while the parent is in a fortified castle)

In the Order of Hermes, a familiar doesn't have any independent rights, and in fact, anyone who isn't a magus has no rights; even mundanes and faeries only have rights to the extent that picking a fight with them causes headaches for other magi. Familiars may be killed to punish their masters; Tribunals have ordered such a sanction. That the familiar in question was human might give a Tribunal pause, but I honestly doubt it in most cases; the majority of Hermetic magi aren't really going to care that this is a person they're killing (because "death of a familiar" is already one step away from a Wizards' March, and in many cases a magus will prefer the March to the death of their familiar).

Obviously, Criamons will vote against any such punishment, but House Criamon opposes murder far more intently than most magi. Other magi who object strenuously to murder exist, and a Quaesitor may have difficulty imposing the death of a familiar (particularly a human familiar) if his Tribunal has a pacifist bent*. Quaesitores in such a Tribunal will probably go for lower punishments.

*Though it's unclear from the Guernicus House rules on Tribunal, here, whether the Wizard's March after three refusals of the Quaesitor's punishment is automatic, or whether it requires Tribunal approval. If a Tribunal is in the weird space where they'll find someone guilty but not guilty enough to March, though, no sane Quaesitor would force a March through.

Loss of the familiar doesn't necessarily mean death of the familiar. Houses of Hermes: True Lineages details the spell that breaks the bond between familiar and magus, and once broken the familiar flees from the presence of the magus. So while they might pause to kill a mundane, human familiar, they will not hesitate to use the spell. Of course, that spell requires the cooperation of the magus (and I'd argue the familiar in the case of a human, certainly the presence of both) and they can evade that punishment by simply not cooperating.

There are numerous story possibilities to be mined here, and not all of them involve the death or loss of the familiar. It could very well be that the charge was trumped up and the vote to convict was forum shopped to a lesser, but still legal, Tribunal. It could be that the familiar is a diabolist and had corrupted the magus and the two were hatching diabolical plots together. And on and on...

Indeed. We were mostly arguing "can a Tribunal issue death of a familiar as a punishment if the familiar is human?" and my answer was "yes."

It'd be a much harder punishment to execute effectively if the familiar was a daimonic aspect, of course. What's the Tribunal gonna do, invade the Twilight Void?

What they can do is be reasonable about it, and break the bond.
Soft Law vs Hard Law.

I agree they could execute a human familiar as punishment, I simply think it would affect the process, and perhaps make the taking of a human familiar more controversial. Also what happens if the familiar is of noble blood? Does that make taking or killing them interference with mundane society, and if so does it apply to both or just one? After all a nobleman who is a familiar could be expected to live much longer than normal, though they would presumably be of much less use in the lab...

Well, if you're taking a noble familiar at all, then either it's the equivalent of (for instance) abducting the Gifted child of the King, or it already counts as a permanent alliance with a noble (which is interference pretty much by definition).

In the former case, it's legal, but the familiar can't expect to call on his noble status to protect him, any more than a Hermetic magus of noble blood could retreat to the king's court and become a court wizard, and then yell Interference With Mundanes as soon as the Quaesitores objected. The familiar is "the property of a magus," not "a nobleman," and if the noble's family objects, the Order will stand against the nobleman in question.

In the latter case, the Tribunal probably will insist that the bond be severed immediately, but will give the nobleman in question a modicum of protection against a Death of the Familiar order. That said, I as a Quaesitor would interpret the oath against interference with mundanes as more of a constraint upon individual magi; if a Tribunal as a whole decides that a nobleman is a problem, then they can declare him an enemy, or if he's a familiar order his death.