a) Code tags come in pairs - an opening tag that says start this here, and a corresponding closing tag that says end this here:
Your page is misbehaving because in some cases you added the opening tag but forgot to add the closing tag or added an opening tag again instead of a closing one.
b) In your first reply you said you want the 'traditional' book look, so I assumed you meant you want it throughout. Now you're saying you want a mixture of the two styles. In that case you must use my other solution ("single line indents"). So:
Delete all the code you have added. For regular paragraphs (blank line, no indentation) enter a paragraph break with no code at all. For old-style paragraphs (indentation, no blank line) enter a line break then enclose the first word in this code: