[enuddleyarbl]

If you make a testing copy of the epub (perhaps in a test Calibre library), you could strip the css out of the stylesheet, copy the book (with a different name) over to your Kobo and see if the narrowness goes away. That, at least would let you focus on the css and not something else.

I've gone through that stylesheet you posted, removed everything that's not obviously table related, added some of my generic stuff in at the top and posted it below. You could try deleting everything in your test book's stylesheet page and copying mine in. Then go to the Tools menu and Remove Unused CSS Rules, then Fix HTML Files - All Files and then Beautify All Files. I'm pretty sure that will remove all the no-longer-present classes in your text. All the fancy stylings will go away, but it should be semi-decent.

@page {
  margin-top: 0;
  margin-left: 0;
  margin-right: 0;
  margin-bottom: 0;
}
body {
/* Basic Styling for BODY Sections of a File */
	height:	100%;
	margin:	0;
	orphans:	1;
	padding:	0;
	widows:	1;
}
* + *	{
/* Lobotomized Owl Selector - what to do any time two objects follow each other - limit to margin-top */
  margin-top:	0.6em;
}
h1, h2, h3, h4 {
/* Basic Chapter Heading Styling */
  page-break-before: always;
  page-break-after: avoid;
  text-align: center;
}
h1 + p, h2 + p, h3 + p, h4 + p, hr + p, ol + p, ul + p, .chapquote + p, .headline + p, .letter + p, .note + p, .poem + p, .placard + p, .song + p	{
/* Non-indented Paragraphs Styling (for paragraphs that follow something and shouldn't be indented) */
  text-indent: 0;
}
h1 + p::first-letter, h2 + p::first-letter, h3 + p::first-letter, h4 + p::first-letter, hr + p::first-letter, .chapquote + p::first-letter	{
/* First-Letter Styling (for the first letter following certain things (IOW, dropcaps)) */
  font-weight: bold;
}
h2 + p::first-line, h3 + p::first-line, h4 + p::first-line, hr + p::first-line, .chapquote + p::first-line	{
/* Make first line of first paragraphs smallcap */
	font-size:	1.05em;
	font-variant:	small-caps;
}
hr {
/* Thematic break to use 3 asterisks between parts of story */
  margin-top: 0.9em;
  margin-bottom: 0.9em;
  border: none;
}
hr::before {
  content: "***";
  display: block;
  text-align: center;
}
p {
/* Basic paragraph styling */
	display: block;
	text-indent: 1.2em;
}
blockquote	{
/* Blockquote Styling */
	display: block;
	font-size:	small;
	font-style: italic;
	margin-left: 1.7em;
	margin-right: 1.7em;
}
.calibre14 {
  display: table-row-group;
  vertical-align: middle;
}
.calibre15 {
  display: table-row;
  vertical-align: inherit;
}
.calibre16 {
  display: table-cell;
  text-align: inherit;
  vertical-align: top;
  width: 3.45in;
  padding: 0 5.4pt;
}
.calibre17 {
  display: block;
  font-family: Arial;
  font-size: 1.125em;
  font-weight: bold;
  line-height: 1.2;
  page-break-after: avoid;
  text-autospace: none;
  margin: 12pt 0 3pt;
}
.calibre18 {
  display: table-cell;
  text-align: inherit;
  vertical-align: top;
  width: 49.5pt;
  padding: 0 5.4pt;
}
.calibre19 {
  display: table-cell;
  text-align: inherit;
  vertical-align: top;
  width: 135pt;
  padding: 0 5.4pt;
}
.calibre33 {
  display: table-cell;
  text-align: inherit;
  vertical-align: top;
  width: 0.75in;
  padding: 0 5.4pt;
}
.msotablegrid {
  border-collapse: collapse;
  border-spacing: 2px;
  display: table;
  margin-bottom: 0;
  margin-top: 0;
  text-indent: 0;
}
.msotablegrid1 {
  border-collapse: collapse;
  border-spacing: 2px;
  display: table;
  margin-bottom: 0;
  margin-left: 54.9pt;
  margin-top: 0;
  text-indent: 0;
}