Hey all, i figured I would update with the solution that we came up.
We noticed that when converting xdp to html, the best way to maintain structure was by using tables. Subforms convert to divs, however email clients do not always render divs properly, so we stuck to using tables to structure our forms and also to control the layout of our forms. However, the layout got messed up when we did the conversion, so the solution that we came up with was to use the renderHTMLForm service with the option of nostyles, and to add the styles in manually.
Essentially we noticed that all of the names of complex objects (objects containing other objects, ie subforms, tables, table rows) that we created in the form would be carried over into the created html, with some additions.
So if we have a table called Details, which was in a a subform called Content, it would be converted to a table with id=NContent_0.NDetails_0.
I'm not sure what the "N" denotes, but the 0 at the end is the index of the object, so if you have repeating elements, the index would be noted at the end.
At this point, ideally, we would have liked to user the renderHTMLForms to import a css file in to use, but it puts the styles inbetween the head tags, which again is not great for email clients. So instead we wrote a custom component to insert it the styles from the css to inline (there are lots of libraries to convert css to inline, sorry wouldn't be able to tell you which).
The issue at this point was to know where to put the styles. Thats where the ids come in and why we used tables to structure the form. We were able to style the different parts of the form by placing them in the tags, matching them up with the appropriate id's. We used a config file to map the styles to the id's to try and automate and reuse as much as possible.
Not as elegant as i would like, but unfortunately the only way that i could think of to resolve the issue.