How can the trail be made to start at (or from) the "parent page"?

That is the first trail markup should be visible on the "parent page". In the example below the next and prev pages should show as (arbitrarily) the first and last pages on the trail (excluding any loop entry).

<|[[{$FullName}]]|>

Why can I not get the Path WikiTrail ^|[[TrailIndexPage]]|^ to function as expected?

TrailIndexPage contents:

*  [[Top Level Page]]
*  [[Level 2 Page]]
*  [[Level 3 Page]]
*  [[Level 4 Page]]

Level 4 Page WikiTrail code added:

%rfloat font-size:0.8em% ^|[[TrailIndexPage | Index]]|^ %%

Expected output:

Index | TopLevelPage | Level2Page | Level3Page | Level4Page

Actual output:

Index | Level4Page

The trail only ever displays previous page | trail page | next page. In your actual example you are at the first page of the trail, so there is no previous page. To achieve what you want will require a page list. simon? July 18, 2009, at 06:15 PM. To always show a next and previous page, have a look at circular-trails. gb? February 08, 2010, at 12:00 PM

How can I modify the linkformat displayed using <|[[IndexPage|+]]|>. I want to have a display like this:

< PrevTitle | IndexTitle | NextTitle>, but I get sth. like <PrevGroup.PrevName | IndexTitle | NextGroup.NextName >, but I don't want the FullNames.

In the bulletted list of links, use [[Group.Page|+]] to get the page titles, both in this list and in the trails. --Petko? January 03, 2010, at 09:10 PM

Where can I change or remove the div and p wrapped around a trails span (for example the one on top of the PmWiki.BasicEditing)

This trail in the documentation is in the GroupFooter, and it is included as a section into the GroupHeader. You can edit these pages on your own wiki. See GroupHeaders and IncludeOtherPages for more information.

Why isn't simply the trail itself a div with the CSS class so everyone can style them as desired on every page ? Why use span and why wrap this in even more markup ?

I think there is no good reason. Layout should be a issue of CSS and only CSS. These wrapping containers (without any CSS class and - in addition - with hard coded style attribute - uuhha) only make it hard to change a tails design.

It is wrapped in <span class='wikitrail'></span> to allow people to use it inline. You can display it as a block and style it differently by adding to pub/css/local.css something like:
span.wikitrail { display: block; border: 3px dotted pink; background-color: yellow; }