a) for db lists, ID Field Name and Value Field Name indicates being able to use a different display and record value - is this correct, and if so; what is the syntax to this?

You can either write your own hql query, or you can specify a classname and two fields that will be selected, using the value, label and classname properties. So, how this works:
- leave the Query field empty
- Select a XWiki Class name (like XWiki.XWikiUsers)
- Select two object properties as the value and label fields. They will normally be used in the hql string as StringProperty-s.

b) for db lists again, there is an Xwiki Class Name field - what does this do? Entering a valid class name does not seem to alter the behaviour in any way.

See above.

c) for Strings/Dates/etc there is a Validation Regular Expression (and associated Validation Message) field; is this intended as a validation expression for the field value? If so, is it active? - so far it does not give any message (provided or otherwise) when used.

It should be working, but I don't know how. I didn't use it.

d) For list types, there is a new entry called Display Type - changing this does not seem to make, eg a dropdown list into an input - is this what it is intended for, and if so; what restrictions are there in the underlying code? Can a select list be changed to a text box with this field (assuming data already entered stays the same)?

This should work, and the effect is that in inline edit mode, the list can be displayed as a drop-down list, a list of radio buttons (for multiple-select = false), a list of checkboxes (for multiple-select = true), or a simple input field.

e) Tooltip and Custom Display: Is this an automatic field render, or must these fields be initiated in some way?

These fields are not active yet.