The HtmlAgilityPack is relatively new to myself but logically 'OptionsCheckSyntax=false' would work. The problem you seem to be running into is your use of '<' and '>' characters.
These characters are used as mark-up for the document itself. Try replacing them with "& l t ;" and "& g t ;" (without the quotes and spaces)
e.g. <extract> Nullable & l t ;int & g t ; c = null; </extract>
Haven't had the opportunity to try this myself but by all indications it should work. Added spaces as browsers automatically convert them to '<' and '>'.
Reference: http://www.w3.org/TR/REC-html40/charset.html#h-5.3.2
Regards