I believe Qt 4.8 is actually used in the latest versions of Sigil 0.5.3 and if Qt 4.8 supports as much of QtWebKit 2.2 as you say, then the Sigil code base can certainly help get you going. Sigil is cross-platform and written in C++, and very readable and the build is easy to do too. So someone well versed in C++ should have no trouble working with the Sigil codebase.
I think you will see many of epub3 features be supported in Sigil as QtWebKit support improves. Helping to move that forward is definitely something doable for a good developer and I am pretty sure that user_none (the developer/owner of Sigil) would accept patches that add features for some specific future version of Sigil.
That is of course his call.
But I do think the Sigil code base would be a big help in pushing your project forward. Talk to user_none and see what he thinks. Perhaps he can set up an experimental branch for you to add features too, that will eventually be incorporated back into the main line.