Essentially, all three of these points are normally swatted, as it were, by the same convention of interstellar war.
(a) It is very difficult to raise the troops required to garrison a conquered planet, and as was mentioned in Honor Among Enemies (and will be mentioned again in later books), the Royal Manticoran Army's appetite for trained manpower has gone up steadily as more planets were captured. It will continue to rise, and will become a major source of strain on the Alliance's manpower resources.
(b) You get them to the planet (normally) without taking massive losses because planets are "required" to surrender once an attacker controls the high orbitals and is in a position to "fry the surface." The convention runs something like this:
(1) You can fry the planet with kinetic strikes, and no one on the planet can stop you;
(2) If you fry the planet, you will kill any hostile garrison troops, but you will also kill hundreds of thousands (or more) of civilians on any populated planet;
(3) Indiscriminate (note the qualification!) orbital bombardment is prohibited by the Eridani Edict and any number of solemn interstellar treaties and covenants;
(4) The trade-off for being spared indiscriminate bombardment is an orderly surrender, thus (in the famous 18th century European phrase) "avoiding an unnecessary effusion of blood";
(5) If a planet refuses to surrender when it has lost control of the space surrounding it, then the attacker is allowed to use orbital bombardment to clear his way rather than suffer massive casualties on a point of principle.
As a consequence of these considerations, the only planets whose garrisons fight to the finish are usually those which have no significant civilian population, and even that is extremely rare, since the attacker will usually either ignore the planet entirely (after all, what harm can it do him once he gets beyond the reach of any surviving groundbased weapons?) or else sends in the killer crowbars from space and converts the local ground troops into plasma... in a nice, clean, non-radioactive way, of course.
(c) Once a planet is occupied, most of the occupation forces are little more than local policemen, there to maintain civil order. Generally, the real fighting power of the garrison will be stationed in orbital habitats for quick insertion with heavy combat equipment and supporting kinetic bombardment capability if required. The need to supply them will still exist, but quite sizable ground forces can "live off the land," at least for consumables, when their zone of occupation is an entire planet.
Does that help?