1 ) voluntary transaction fees make no sense to me. Transactions add information to the blockchain permanently. They impose costs on everyone using the sytem (storage costs, bandwidth costs). This should not be free.
There is a voluntary transaction fee being used at the moment. The higher the fee you pay, the faster miners will serve your transfer.
2) a big problem is measurement of velocity. A side benefit of transaction fees is that they will make velocity measures more reliable.
Velocity should not be hard at all to calculate, its all in the block chain. If you are talking about bogus transfers, then your point is correct, but I really don't think those are that big of a problem (and the voluntary transaction fee was introduced exactly to fight this in the first place).
3) Currently miners are paid through seignorage (issuing coins), this is equivalent to a tax on the rest of the user base. Replacing a part of the seignorag tax with a transaction fee tax would not make users worse off in general. (the incidence of the two taxes is different but they are both taxes. more on this later.)
Well, it sounds really bad to "tax" and if this was ever to be adopted, it would need to be accepted by a majority of anarchists and liberal types on this board and they don't generally know that mining is the same as inflating, which is the same as tax. Also people would have a slightly lower incentive to transact, so it wouldn't have exactly the same effect, but yea, almost.
3) Miners will always need to be rewarded. Therefore any velocity based system could never drop the coin generation rate to zero.
Did you see my comment on spreading out the mining profit over time? The idea is, profit now at current rate and a promise of profit at later dates at future rates. Regardless of whether the rate is 0 now, the promise of return on a later date will still be worth something. If you did see this proposal but don't like it, please elaborate.
I expect to see a lot huge increase in the velocity over the next couple of years so I don't think controlling inflation will be a big problem long term. Also it is generally excepted that the coin is deflationary by nature. Short term inflation (sudden price drops) will quickly be eliminated by speculators (especially since they have the source code and can very accurately calculate the probable mining rate at the next period, in real time).
Another tool to further be able to fight inflation would be to calculate a "deflation margin" based on the volatility of the velocity. This just means if the money velocity was volatile last period (prices were volatile), we would allow for more deflation in this period. This would be to ensure that we don't pump out a lot of money this period to hit our deflation target, just to find out that next period the coin is very inflationary and we wish we could take all those coins back we just pumped out.
Ultimately, whether or not your idea is good comes down to whether or not taxing the already existing transaction fees and the "deflation margin" will be enough to cope with inflation or not. And to be honest I am not sure that it is.