Since they all seem to carry the same information I would also give them the same node name.
Agreed. "keyframe" or something like that, with "sequence='0'" as an attribute to replace your numerical node names. Likewise for the children of these nodes - "step" and "stepNo" for their node names and step number, respectively.

If these are documents over which you have no control, then you could try making a call to QDomImplementation::setInvalidDataPolicy() with the argument QDomImplementation::AcceptInvalidData before creating any instance of QDomDocument. I am not sure if this works for both reading and writing with QDomDocument; the docs show examples only for creating documents.

But if you are the one creating these XML documents, then you should fix it to comply with the standard. Otherwise, your files may be unreadable by other programs your users might want to use to browse or display the contents.