For iBooks you need an additional file in your ePUB. If you do a quick search on the forum, you can find it quick enough. ePUB's created with my Word Add-in have this file already present.
Readers that ignore css are not good readers anyway. Perhaps some very lame readers don't support it. I personally use <i></i>.
Font family in the p, h2, etc is fine. You could include !important. Some readers work better with it. I know that some readers (again Nook I believe) overrule certain book styling without it.
With regards to the 'witregel' I can say that it would solve it. In that case you can delete <p class="witregel"></p>.