Probably has something to do with the fact that, while everyone with any meaningful knowledge of the supernatural knows that Fae are vulnerable to Cold Iron, they might not immediately know that the Billy Goats Gruff are Fae.  Harry had to look it up, and get Bob's help (Bob being Harry's version of an extensive arcane library), and Harry is rather more than minimally familiar with the supernatural.

The whole point of the Laws has nothing to do with the corruption of your soul (though it makes a good justification for zero tolerance), it's to limit the amount of power and damage that an individual can accrue.  It doesn't make things fair, it makes them survivable.

There are two sides to the Laws, and one of them has everything to do with the corruption of the soul, and nothing to do with anything else.  The other has nothing to do with the corruption of the soul, and, as you say, everything to do with limiting potential abuses of power by individuals who are immensely personally powerful.

I understand the First Law to be a specific prohibition against magical murder, which is distinct from magical killing.

Your understanding is mistaken.

You presume that killing can ever be a 'good thing', whatever of the reasons.

Is there an official ruling on this?

There is not.

Your suggested houserules have a few potential problems.  Notably, basing the benefits of the amount of time spent feeding rather than the amount of feeding accomplished in that time punishes characters for being good at what they do.

First off, the easiest distinction: the enforcement of the Laws by the White Council and its Wardens is not actually bound to the cosmic truth of the corruption caused by violating the Laws.  The Council is a political agency, and makes decisions for political reasons.

As for the 'killing for good reasons'...before you can reinforce the idea that it is okay to kill 'only for the right reasons' you must first reinforce the idea that there are 'right reasons' to kill, and that the individual is capable of distinguishing them.  And that is exactly the sort of corruption that Lawbreaker represents: that one individual has the right to decide whether or not another deserves to die.

It also allows the wizard to initiate a grapple without first establishing an appropriate aspect, and allows grapples of strength far in excess of normal (which isn't so much a problem in and of itself as it's something that makes all those other problems more apparent and troublesome)

You mean use "Unseelie Magic Power/Control" foci rather than "Entropomancy Power/Control" foci for the entropomancy-as-evocation that Unseelie Magic lets you use?

You could get yourself an Entropomancy Offensive Control focus, but it wouldn't make much sense to do so if your Entropomancy evothaum comes from Unseelie Magic and you already have (or could make/acquire) an Unseelie Magic Offensive Control focus.

It'd be rather like an individual with Ritual:Ectomancy getting themselves a Summoning&Binding focus.  They could do it, but it would only help them Summon/Bind ghosts and the like, since those are the only things they can affect with their magic.