Hello,
It sounds like you are making all kinds of progress.
I have not looked at any of this stuff since May, and I am much too busy with things that are way too interesting to put aside.
I’d suggest you take TiddlyWiki’s empty.html, add to it just the parts you are tying to get working the way you would like, and see if anybody over at Talk.TiddlyWiki can help out.
The things you are wanting, I’m sure they can all be done with the right kind of CSS, HTML, and TiddlyWiki widgets.
For example, editor and preview side-by-side panel’s, each thing needs to be in a div, and those two div’s set to be side-by-side. The preview’s div will have a “display: hidden” style when a “show preview” check box widget is not checked.
Something like that.
Edit text widget with tag=textarea, it will automatically grow in height, I think, when needed. If it isn’t doing that, just get rid of any related CSS that is impacting the height of it.