I may be able to help with John Sewell's gene pool question.  By profession and training I am a molecular geneticist and I have done research on morphological and genetic variation in honeybees (the Asian species, but it is applicable to mellifera races as well).

Some definitions:

Gene: a physical location on a chromosome that contains genetic code.  This code is translated by the cells of an organism into a protein that has some regulatory or structural function in that organism.

Allele: genes are present in different allelic types.  The alleles of a gene code for the same functional protein product, but each allele makes a slightly different variant.  Some have beneficial effects, some have negative effects, most are neutral.  Crudely stated, natural selection "chooses" the more beneficial alleles and "ensures" their survival in the population because these alleles allow their bearers some survival or reproductive advantage over those bearing less beneficial alleles.   A genetic allele is one that has a different code than another.  A functional allele has not only different code, but codes for a slightly different product.

Recessive and dominant alleles.  For the product of a recessive allele to be made by the organism in sufficient quantity to have an effect on the physical structure or well-being of that individual it has to be present in two copies, i.e. one from mother, one from father.  A dominant allele needs to be only present in one copy.

Gene copies:  During reproduction, a parent makes copies of its genetic material that is combined with that of the other parent and is transmitted to the offspring.  You have copies of your parents' genes, your grandparents' genes etc. in your cells.  Along the way, a copy that is passed to an offspring may have mutated during copying (some misreading of the genetic code occurred and was not corrected), forming a new genetic, but not neceassarily a new functional allele.

Gene pool: a theoretical assemblage of all genes that may be combined through mating.  e.g. If one group of bees is separated by an impassable barrier such as a mountain range from another, they are in different gene pools since they cannot freely exchange genes by mating.

Gene flow: the movement of gene copies from one gene pool to another through migration of individuals bearing the gene copies and their mating within the new gene pool with subsequent survival of those new gene copies in the new gene pool.  Simple migration is not gene flow - the genes must survive to become a part of the new gene pool.

Gene pools may be large, that is they have many copies of genes in them (equivalent to many individuals in the population), but they are not necessarily diverse.  If many of the gene copies descended from the same ancestral gene then genetic diversity is lowered.  This typically happens after a phenomenon called a bottleneck.  When a few individuals form the basis for an entire population, for example relatively few bees were the basis for all of America's (used in in the geographic, not nationalistic sense) beehives, the bottleneck effect ensures that the American population of bees - and its gene pool - has a less diverse assemblage of alleles than the parent (European) population.  Robert Brenchley is (mostly) right not to worry about the gene pool in England, since English bees are probably derived from a large founding population that reinvaded Britain after the ice ages and have been living and mutating for thousands of years.  American bees were brought from Europe only a few hundred years ago, and in small numbers, so their founding is more recent than the English bees and stems from a more limited number of individuals.  Both are paltry in comparison to the mainland European genetic diversity, or the incredibly diverse African gene pool(s).

A race is an identifiably different variant that exists in a separate geographic area from other races.  Apis mellifera mellifera is a different race from Apis mellifera ligustica because presumably the Appenines are a significant barrier to gene flow between the two areas.  This lack of gene flow causes a form of inbreeding in the race, enhanced by natural selection for genes that give the bees an enhanced ability to cope with the conditions of their local surroundings.  Mixing individuals from two races causes outbreeding and results in hybrids.  Mostly this increases vigour in the offspring, but it may also disrupt groups of alleles (called a gene complex) that evolved in concert to improve the survivability of those bees bearing those gene complexes.  The result of this is 'outbreeding depression', and results in hybrids being less capable of surviving in either parental habitat than its parents were.  This makes sense if you think about it - take a bee from Africa that has evolved a very efficient way to cool the hive and mix it with a bee that has evolved in Scandinavia and doesn't need to cool the hive that efficiently.  You may end up with a bee that is too efficient for Scandinavia, and not efficient enough for Africa.  Clearly this is simplistic, but it illustrates the point.  Inbreeding in races keeps locally adapted gene complexes together, and is beneficial.  Inbreeding due to a massive bottleneck or improperly managed artifical selection (such as in zoos - or like the famous Habsburg chin) is usually harmful to the organism because it reduces the diversity of the gene pool and allows the expression of recessive harmful alleles, or it causes the fixation of gene complexes in which the individual genes are not harmful, but acting in concert they are.  Natural selection cannot cure this because it has no new alleles to choose.

So - inbreeding due to mismanagement or bottlenecks may be harmful, and the introduction of new genetic material by importation may result in hybrid vigour.  Inbreeding due to natural selection keeping locally adapted gene complexes together is beneficial, and introducing new genes may be harmful.  The case in Africa and Europe to 'let them be and they will sort out their problems themselves" has merit because the gene pools in those areas is diverse enough and large enough that mutation may provide the genetic material for natural selection to act on, or it may already be present and hidden (perhaps recessive) and just waiting for the right environmental conditions for it to be selected for.  In America and Australia, due to their (presumably - has anyone measured this?) low genetic diversity this may not work, because the raw material for selection to act on may not be present.  The populations are large enough that mutation may provide the right genetic material, but this is such a hit-and-miss event that it may be anywhere from days to millenia before the right gene evolves in America to cope with varroa, for instance.

The way I see it, if someone in the world has a bee strain that has a beneficial resistance, controlled scientific attempts should be made to breed it with local bees in the hopes of transferring those genes to the local gene pool.  Then natural selection can be let to fix that gene, or managed breeding can do so in a shorter time period.  We obviously need to avoid the africanized bee fiasco.  The way I see we are right now at the same point in beekeeping as the ancient Egyptians were with cow breeding.  They had the massive Auerochs and other mainly wild cows, and now we have docile milk machine Holsteins that any six-year-old with a stick can chase across the field.  We have a ways to go to get Holstein bees from our Auerochs bees.

Martin Damus