Have you looked into MailChimp in addition to Campaign Monitor. MailChimp’s WYSIWYG (what you called a “front end way”) HTML email editor is about as good as you’re going to find. There’s also a WYSIWYG Macintosh HTML email editor called Coffeecup, but using it is less than intuitive, and it costs money.
There are a number of free and low-cost HTML/CSS WYSIWYG editors available. The trouble is that they’re all mostly intended to build basic websites, whereas the code in HTML emails needs to be constructed somewhat differently for reason Iraszi already mentioned.
The best way to do this is with an ASCII code editor working with raw HTML/CSS code. A very basic and free ASCII text editor is already on your Mac. It’s called TextEdit, but you’ll need to change it from using rich text to plain text in its preference settings. (Plain text is another name for ASCII text.) Then again, if you don’t know how to write HTML and CSS, it won’t do you much good.
How extensive are the changes you want to make to this template you already have? Do you want to really juggle things around or are you just needing to change a thing or two?