perhaps it would be better to recommend the Default OpenACS Tabbed theme
This won't generate tabs for layout managed pages, nor will it display includelets according to the layout chosen for the page (which is why it appears to "work" for you).
Also, the xowiki stuff might display properly on a page with a single column.
Really, content displayed in includelets shouldn't be further sub-divided by embedded HTML, because results are going to be hard to predict and may work or fail depending on the layout/theme selected for the page/portlet.
You could display a "cover page", if you will, in the xowiki that just has summary information then link to the full page (when displaying in a non-managed application page, you'll get the standard one-column layout that you see in an ordinary subsite using the default theme).
BTW, if all you want is a simple way to have editable text in an includelet, check out the "content-includelet". It provides a simple way to provide versioned HTML with the typical controls to set a particular version "live", to delete, edit, etc. It also provides a simple way to do variable interpolation (so you can grab stuff from the db and have it appear in the text). It's been very useful for me ...
