Look, I know you're just trying to help and thanks for laying it all out for us (obviously you've done a lot of research), but I guess the problem is that many of us don't want to do calculations and become responsible for clients' withholdings. It's like doing the tax return twice; but they won't want to pay you very much for a W-4 form.
Of course I'm beating a dead horse here because IRS will do what they want, which is probably to collect more money up front. Software (which I don't want to buy) can make it easier, but it won't absorb the blame when taxpayers give you understated or wrong estimates and owe money in April. We had a perfectly good, simple, and practical W-4 which was the taxpayer's problem - not ours. If short, all they needed to do was write in hold X dollars per week more next time. Now (along with the 1040) it has been blown all to hell - they've created a problem and handed it to us. Maybe the "estimates" will work out; maybe they won't, but clients will have a handy target when they don't.
Okay; rant over, but Lion's right - this isn't simplification. It's more in the direction of the ever-expanding 8867 whereby we became IRS' "partners".