A nested, optional subsequence is not straight-forward mapable to entities and lists of entities as currently lists can contain only one type of entity. To workaround this we would need to create a rather complex construct in the mapping and also increase complexity in the modeler UI. Considering that optional nested sequences are not that often used (at least, we don't hear this requirement that often), this is not high on our priority list.
Currently the alternative is to transform the xml into something that is mapable, e.g. by using an ESB and XSLT, create a custom java action. In the future you will be able to return the full response xml as a string from a webservice call, and then you build some custom logic in a microflow for dealing with unsupported constructs in xml.
Summary: it's a lot of work, it's not much used, and workarounds are available.