>Third, when he creates a "ley line" does it connect all the
>together in a domain, or does it just come from a general area
where all
>the sources coalesce into one?

As I understand it a Leyline connects two provinces in a straight line.
These do not have to be neighbouring provinces although the power cannot
be tapped in any province in between the two provinces unless a hook-up
is also made.

i.e. The ley-line acts as a power line between the two provinces and can
only be accessed on either end or somewhere there is a sub-station.

This means to build ley-lines around a whole domain may require several
lines and several hookups.

A problem you may encounter is that players want to build a ley-line
leading from a far edge of a province just so that it can go through
more provinces, always define the location of the source way before
that player actually gets to build ley-lines.

You asked about guarding a source, many wizard players build their
tower close to their biggest source just to keep an eye on it. Building
barracks and other buildings to house guards close to the source may
have an adverse effect on its power (DM choice) especially if clearing
forest or polluting streams. Guards tramping around a site day in day
out will soon lead to source damage.