Ok didnt feel the need to read the whole thread but read enough to get the idea.
My parents own a tyre store here in Townsville which i used to work at. The law states that a spacer to be legal MUST be fixed to either the hub or back of the wheel. So slip on spacer ARE illegal. As far as wether spacers cause shakes or not, yes 99% of them do. but sometimes it is fixableish, but takes alot of time and a shit load of mucking around, and balancing the whell 300 times is not going to fix it. The reason being is that they (spacers) make the whole assembly (of tyre and wheel) out of round, if you jack up one corner of the car and spin the wheel you will get a high spot, this IS what causes the shake 99% of the time. But please also note that poor quality wheels that require hub rings to take up the the area where the centre hole in the wheels is not a snug fit will do exactly the same thing, and believe it or not so do poor quality tyres. Trust me i used to fix these problems every day of the week. Also there is a miss conception the wheel alignment fixes shaking problems....WRONG. A wheel alighment is there to adjust your cars wheel geometry ie caster, camber, toe, SAI etc hence the name wheel ALIGNMENT not wheel BALANCE!
Cheers Daniel