Mr Cucumber,
I don't know what else I can tell you that will be of use. Your thought process hasn't got any obvious errors or holes in it as far as I can see.
Very few consultants do any kind of commission or results based work, usually because marketing consultants are usually pretty useless, and if they worked commission only they would starve to death.
I personally don't even like the term marketing consultant, as nobody who is looking for one even knows what marketing is. Even the Chartered Institute have started referring to marketing and business development, rather than just marketing.
If you are good at what you do, working on a results based fee opens doors that would normally remain firmly closed. You will get much more interesting work, and get to educate business owners along the way.
I have been chatting to a web designer from this forum on and off for months. Whenever he is a bit short of work he gets in touch, refers to a couple of my posts, and asks me if I could help him get more work. I tell him yes, and then he thinks about it for a few days and talks himself out of getting help. The reason we never close the deal is because he expects me to sell the idea to him, but I cant be bothered. The problem is, he has no idea what to expect from using my services, and for a couple of days money, I'm not going to waste my time explaining it to him. My point really is that you need to make sure that if you work on a results based fee structure, you make your offering completely clear, so you spend your time working and earning money, not acting as a salesman trying to persuade people to sign up. Equally, you need to separate the genuine leads from the time wasters early. That is really what the initial fee is all about. I even offered this guy help based on the idea that he would pay me for one day, and if it didn't generate more than it cost I would pass work to him for more than my fee. I still haven't heard back. Sometimes, even if you offered to pay someone to make them money they would still dither about not knowing what to do.
You are right about a sliding scale for fees. Working for a set fee per day is ok for the initial research, but you really deserve a piece of the action if you are working on results. Obviously this needs to be tailored to the business. Using the example above, I could generate enough work for the web guy to have 500 hours work per week, but what use is that to him? He works on his own, so instead I would concentrate the marketing plan around finding him the most lucrative work for him to spend his time on.
I specialise in marketing any business offering a service. This is a balancing act as too much work, or growth at too fast a pace, can kill a small business just as quickly as no work at all. Don't underestimate the responsibility that goes with messing with someone's business.
The problem with results based fees is that they are just that, results based. As you say, a lot of what I do will ensure that the business makes more money for years to come. Proper strategic marketing is always going to be like this, and there is very little you can do about it (unless you decide to be a pretend marketing consultant offering telemarketing, social media stuff, or help with advertising

)
As far as predicting future unaided sales, its really down to experience and negotiation. The best method I have found is find out what level of income they would be happy with (as long as its realistic) and negotiate yourself a percentage over this amount. You do need to be pretty confident of your abilities to do that though.
The other thing you need to consider is the ability of the person you are going to be working with. Don't fall into the trap of working for people who are going to blame you for the failure of their business, even when they roll out of bed at 10 am to do a couple of hours before lunch. Clients need to deserve a results based fee.
Another thing you need to watch is not seeming too good to be true. A while ago we had a good discussion on here about dog walking businesses. I said that it is easy to make 30 grand a year as a dog walker, with no staff. Because everyone on the thread said it was nonsense (even the dogwalkers) I was seen as probably telling porkies. Now, I would be prepared to back that up with a results based fee for anyone who wanted to give it a go, but I have never been approached because it seems to good to be true. Sometimes you need to take the shine off of your offering a bit, in order for people to see the value in it.
I could go on about this all day, so should probably leave it there for now.
Sorry to waffle on, I hope somewhere in that lot is something of some use.
If you have any specific questions I'd be happy to help.