I posted most of this elsewhere. I don't mean to be disrespectful to you or your parent's experiences (I'm just a "newbee" myself), but a good bit of what you write isn't quite correct.
In summer, a typical worker bee lives for about 6 weeks. 8 weeks, maybe 10, if she has one of the rare posts of guardians at the bee colony's entry, or is one of the even fewer bees that feed the queen.
No. Most worker bees go through a predictable lifecycle. See, e.g., http://www.clemson.edu/extension/county/oconee/programs/beekeeping/Honey_Bee_Life_Cycle_in_Pictures.pdf
1-2 days old: Cleaning duty
3-5 days old: Feeding older larva (nurse)
6-11 days old: Feeding younger larva (nurse)
12-17 days: comb maintenance and production (wax)
18-21 days: guard bee duty
22+ days: field bee (foraging)
It's relatively rare for a bee to have only one duty over its entire lifespan, though this can happen. Sometimes phases are skipped in the spring if a colony really needs foragers, for instance.
Bees literally work themselves to death. The replenishment rate is, during summer, 100%; this is taken care of by the queen.
More than 100%! Colonies expand rapidly in the spring and into summer.
A typical bee colony has between 10,000 and 40,000 bees in high summer, then goes into winter with about 1,000 bees, clumped around the queen to keep her warm, and comes out of winter with 400 to 600 bees.
40,000 is on the low ends of most estimates I read for summer population. Some estimates are up to 100,000 bees!
When you buy "package bees" to install in a hive, the typical size is 3 lbs of bees. This is over 10,000 bees, and is a small "starter" colony. So, I think your estimate of 10,000 to 40,000 bees for full, established summer population is very low.
Finally, FAR more than 300-400 bees survive the winter. I do not think a colony that overwintered with only 400 bees would be viable.
We are talking about apis mellifera carnica here, the so-called Italian bee, which is the variety most commonly used by beekeepers.
A. Mellifera Carnica is the "Carniolan" bee. It's another somewhat common breed of honey bee, but nowhere near as common as the Italian bee--Apis Mellifera Ligustica.
An entire colony dying in spring or early summer is, normally, an extremely rare event, and indicates either an epidemy, or severe poisoning.
Colonies death in spring is not at all uncommon. Colonies are actually at very great risk in the spring. When the queen starts breeding again and the hive starts growing, resources that have been stored since the previous summer are used up very rapidly. The bees leave the cluster and start moving around. A few bouts of bad spring weather that disrupts the early nectar flow or an unexpected hard freeze can destroy a colony that survived all winter long. If a spring colony has depleted all reserves and there's a nectar death and cold weather, things go bad fast!
Colony death in summer is more unusual.
Varroa mites are a known cause, but are a largely contained phenomenon now, at least in professional bee-keeping circles.
Somewhat. Effective treatments have become available only in the last several year. Near constant monitoring is still required.
Feral bees have come close to being wiped out nationwide. This is by no means a problem that is linked to just the much vilified commercial beekepers.
What remains, is ... poisoning. Neonicotinoids or something else.
Speculation and hyperbole. There are thousands of reasons hives can die. I do not believe the neonic connection has yet been proven, though with Europe banning, we should have some good data coming in over the next few years.
