What about using Google Docs API? You could use either their word-like doc or a form to get the data you need, and then present that internally.
Not sure if this meets all of your requirements, or is an available option.
For our company, we have a few tools that utilize Google Docs. We upload data dynamically to them for specific needs.
Based on your requirements, maybe it's best to just write your own. I haven't created a Rich Text Editor. But it looks like there are quite a few tutorials online. Here is a basic tutorial for a rich text editor. It's using javascript, HTML, & CSS. If you prefer to not use js, then you may need to look for other tutorials.
This isn't the most glamorous solution, as it looks like the users view would be HTML. I'd think you could have it updating dynamically off to the side with an actual rich text view (similar to how Stack Overflow has theirs below an answer or question being written).
Update
Over the weekend I was exploring HTML5's contenteditable
attribute, I came across an editor that builds off of that called Aloha Editor. It's a WYSIWYG type editor. But if that's something that you desire for your clients, than this would probably be a pretty simple integration. I have yet to use it, but it seems like it would be a great fit - if you decide to go the route of building your own editor.