The tag lines you are quoting are all scenarios -- possibilities. Just as is the picture you paint in your 5th paragraph. It seems you have had a rough ride, some do, many don't -- circumstances all differ as, I have to admit, do IP /firms.
Some people DO pay back around 10% of debt (they are also, as you have been, forgiven the contractual interest for the term as well). For most it is nearer 30%. And some people pay £80 a month .... you earn more, you pay more.
The object of an IVA is to pay back as much as you are able, NOT to pay back as little as possible. You borrowed the money, so you try to pay it back.
You are right, it isn't, by any means a bed of roses and, in some cases can be a rough ride and, yes, it is YOUR IVA ,,, after all, you are the one who suffers if it goes pear shaped, so you need to understand it and challenge the IP if he or she strays from the terms. but, yes, like any contract, you are still bound by it's terms.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014