What you could try is loading the mobi book in calibre, then convert to epub using the GUI. Since you want to fiddle with the book, getting a single large HTML file might be convenient. To obtain this, click the convert ebooks button with the book selected, go to the 'epub output' option, check the 'do not split on page breaks' box and set the 'split files larger than' to a large value (say, 9000 KB). Let Calibre convert it. Find the converted file in explorer and rename .epub to .zip, then open with whatever archive software you use and extract the large HTML file (and possibly the css file), ignoring all the other files. Remove the book from Calibre's library and so on as its useless and edit the HTML to your heart's content.
I recommend running it through
HTML tidy and if you are comfortable using python,
Ahi's pacify.py before you do any of your own editing.