You will have to worry about formats beeing over written.
Ex.
<p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. ____Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
<p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua._____ At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
Ends up with having the part between the “_” over written. Even if you edit it as plain text, you will loose the closing and opening paragraph, so two paragraphs become only one.
Everything else you asked for can be done with filters on the_content or get_the_content (please take a look at the codex for further explanation). You’ll need to use str_replace or preg_replace (native php functions, please see php.net for explanation) inside those filters.