Any cause is possible, but I think different component failures occurred in each case due to different stressers or root causes.
Cell phones can be sensitive to rain or >90% RH at high temp if no watertight seal on critical connectors. I also depends on amount of residue in solder flux & dust ingress also if they use "no clean process" or "with clean." Some gaps under chip are hard to clean. The internal ribbon connectors can also become displaced when the phone is dropped. Also weak solder joints or heavy parts on board such as display can break joints if cell phone is dropped. It is hard say what stress phone has seen, just maybe.
Mac Air magnetic DC plug is excellent but is hard plastic and the DC wire is soft flexible wire and jacket. Sharp bends on wire at plug and frequent pulls are most likely stresser. I happen to own an 1 yr old Mac Air and have 2 different DC chargers and both are weak in this strain relief design. They opted to make plug too easy to connect with magnet and disconnect by yanking and inadequate strain relief on hard plug to soft jacket interface. I wrapped both of my plugs with stiff tape ( not duct tape) and even with that one is failing at the wire plug interface. ( I am hard on mine )
Graduated stiffness so there is no sharp bend in wire. Sharp bend is anything with bend radius < 20x wire jacket radius is OK but with frequent flexes. premature failure will occur. They use many awg 48 stranded wires thinner than human hair to make very flexible cable, But it can break without obvious physical symptoms.
It may be too late to fix. I just noticed same Mac Air DC charger failure symptoms on one of my charger as yours and added a few wraps of red Tuck packing tape around wire next to DC plug. So far so good, but next time I have to be more gentle. No more accidental plug yanks. ( which disconnect so easily with magnetic plug but overstress wires.) It is deceptively smooth, a good design but with a fault for heavy users who are mobile with charger. Stationary chargers would be even better, if you could.
The same is true for headphone, earbud and microphone cable and cell phone charger cable users.
If they stayed on back of desk, between uses,they might last for ever. But if stuffed, moved back and forth daily with mobile useage, maybe last < 6 months. It depends on strain relief near both ends of wire and frequent flexes < 20x thickness.
The MTBF stress factors for cable depend on bend radius ratio and frequency of radial and axial stress per day.
Cell phones are design to withstand the international standards for ESD > 15kV.
If you get a lot of lightning in your area, get a few 25 cent coin size MOV's for your extension cords and make a plug with one. Your power meter in the home will suppress transients> 6kV, but yet the industry only certifies consumer equipment to 1500V or maybe 3kV since 6kV costs more with line filters. But cheap protection is an MOV rated to clamp just above your max line voltage. It is good for 10 or 20 large surges.
**You do not need a fancy spike meter unless you are really keen. But you may want fancy line filter or just a cheap one.
A cheap spike meter is ac powered piezo alarm with the switch turned on but the wire broken and splice with exactly a 1mm air gap and sealed with tape. THis will yield a 1kV arc gap +/-25% 1kV/mm for smooth gap. So when line transients occur > 1kV you will know you need an expensive $25~$50 line filter with 1500V protection using CM choke, diff choke, MOV, TVS and gas tube with current limiter.**
MOV's are cheap 25 cents typ.
DC chargers may be prone to breakdown with spikes burning enamel insulation but more often fail due to self-heating or lack of free air flow (stuck under blankets) and quality of vendor with poor vacuum processing of injection molding.