Your PM array problem is easy to fix:

Just replace your message textarea with this:

<TEXTAREA NAME="message" ROWS=20 COLS=60 WRAP="VIRTUAL">$message[message]</TEXTAREA>

In fact, you seem to have lost quite a few of the correct variables for the contents of your textarea elements in your templates. Review the default templates and check that you have the correct variables inbetween the <textarea> and </textarea> tags. This will fix your editpost and priv_sendprivmsg problems.

Finding a workaround for IE versions less than 5.5 having no array.push() method is going to be a little more tricky...