Thing is most players will not want to be in (and in turn not play in, if your the GM) games where their characters are nerfed so heavily, especially when your investing so much refresh into the powers to begin with. If you were to insist on that House Rule (as I think your hard and fast ruling would basically be just that) I would say that giving a +1 refresh rebate would be only fair as a result for you handicapping those players so heavily when they /are/ paying for them full cost.
Yes, this is a house rule, as is saying that killing accidentally is impossible, as I've said several times before, neither is supported directly by the rules.
I don't think it's nerfed at all. Evocation and Thaumaturgy are IMO inarguably the most potent expenditure of refresh in the game, and I think that the risk of law breaking is something that does a good job balancing them. A wizard, without any obvious weapons and without preperation can throw a Weapon: 2 attack over a zone, that's the equivalent of shooting everyone in the area with a pistol. That's VERY powerful and something that pretty much only Wizards can do and can do so with no chance of becoming a law breaker. Alternatively when not dealing with a mortal a Wizard can throw a Weapon: 5 attack or higher at one or multiple targets at range with is something only replication perhaps by a super strong character throwing a truck, and again he can do this without preparation and completely by surprise.
If I was playing with a group who would not play unless Wizards were even more powerful, I probably wouldn't want to play with that group anyway to be perfectly honest.
Don't matter, the way the Lawbreakers are written is that you have to directly kill them with magic for it to be an LB. Thats why setting fire to a building and them dying is a no no, but binding them in place and cutting their heads off with a sword is not.If they die because its a secondary effect (walk in front of a speeding car for example) then no LB, but if you were to say cut their parachute, or blast them with flame you do get it.
I disagree, and again I think that the designers disagree as well. If pushing someone off a building is sufficiently direct to gain lawbreaker, then lighting that building of fires is so as well.