I did find the idea for tweaking Calibre input & output prefs to be useful.
Finding the ability to tweak the file in Calibre is what got me to finally start providing e-pubs. One thing I did find, when I first started, was I could create e-pubs that validates at the IDPF Validator ( http://validator.idpf.org/ ) but got rejected by the checker / validation program at Lulu (more on that later).
However, at that time I wasn't using Heading 1 and Heading 2 for the chapter and sub-chapter headings in the document file. I went through a document and redid all the headings using the Heading 1 and Heading 2 codes and created a new PDF, and it went through OK. The only other trouble I had was when I created an e-pub and tried to load it up straight away, the validator didn't like a publication date in its future. That's now fixed. Sometimes I have to load the e-pub file a couple of times before it's accepted, I suspect the problem there is related to a data corruption during the upload - maybe the electrons got too soggy crossing the bottom of the Pacific Ocean (you may laugh now).
Now, back to the Lulu validator. When I first started loading e-pubs at Lulu the process was very much like when you loaded a PDF file for the print books. On one of the early screens you had three choices - sell via the market group, sell via Lulu only, restrict to only yourself. At that time when I loaded an e-pub as sell via Lulu only it didn't run a validator program over the file, but if I selected to sell through the market group it did run the validator over the file and was sent to all the members of the group. That showed the validator program was being insisted on by the other members of the market group (Amazon, Apple, Kobo, B&N) and not Lulu. A few months later the upload system for the e-pubs changed and the choice screen was removed, but all e-pubs being loaded were then put through the the validator, and at the end you got a screen to choose which of the market group you wished to sell through - you could pick and choose. If you pick Amazon you get another page listing a large number of other requirements the file must adhere to before Amazon would accept it - one is the story can't be on Amazon already.
Another issue I've had with e-pubs was at one point the metadata was listing me as Mr Ernest Bywater, and Apple were getting their nickers in a twist about that, and I had to have the Mr removed from the name before they'd accept the file. This was after it was passed through the validator. Investigation found my account at Lulu had the Mr in its name, changed the account name and all was well. If Apple had been the main force behind the validator they'd have had that as part of the check routine, instead of running it through another check at their end. And since the early use of the validator only came when using the market group, it wasn't being pushed by Lulu. I later confirmed this when i started having the trouble with posting e-pubs with a date in their future, a complaint to the Lulu Support people got a response of them saying they'll pass it on to the market group member who manages the validator software.