Don't the two hot legs need to be balanced if it's line to line to compensate for the fact that there's no neutral?
No, if there is no neutral then all the loads must be connected across phase to phase. In US terms this would mean that there are no 120V loads that need to be balanced.
I'm not convinced you have two hot legs from a US style split-phase supply.
The Chinese standards seem to be pick and mix of other countries standards. Note that Red=Live, Blue=Neutral, Green/Yellow=Earth is a valid colour combination in Australia.
The four "two-pole earth leakage breakers" (tengen DZ47LE-32 C16) are marked with N for Neutral at one connection.
I would therefore, naively perhaps, expect the blue wires to be neutral and to have zero or very low voltage with respect to the green/yellow ground wires.
I don't see any normal bus connectors, it seems the installer used red wires to construct buses live and instead of the neutral terminal that would be found in a UK consumer unit.
CE marked C16 circuit breakers should be 16A breakers with a "normal" sensitivity range.
The blackening of the top screws of the old breaker is also concerning. Is too much resistance on the wire or screws/breaker a possible cause?
The clamping screws were not done up tight, the cables were not inserted correctly or the switch was passing too much current. Or all three.
Example, for comparison purposes, of an old and unusually neat UK equivalent showing copper bus bars at bottom connecting live feed to single-pole breakers, terminals at top for neutral and ground (these terminals are sometimes also called bus-bars). Note the copper bus bars normally have a plastic shield, and normally there is a cover that shields all the wiring with holes for access to the breakers only.
- Image by RF Lighting
Since China is not the UK, there may be entirely different exemplar Chinese installations that bear little relation to this. The question shows an installation where every breaker and switch has two poles. This means there is no need for a neutral terminal connector strip. However the neutral wires linking adjacent "earth-leakage breakers" form an improvised bus.
What you may be seeing is a flush-mount version of a spa panel or enclosed breaker
From your description, I would reckon that what you are looking at is something akin to a "spa panel" disconnect or circuit breaker enclosure, only in a flush-mounted configuration (vs. the surface mounting typically attributed to spa panels and enclosed breakers). These are usually 2-4 space loadcenters, with 70A to 125A bussing; larger versions use a breaker with line and load lugs instead, bolted to the box, and are called enclosed circuit breakers.
Best Answer
I suspect part of the problem may be AFCI, GFCI and other recent advancements. A simple panel is totally passive - as long as there are no loose connections causing sparks/arcs, and actual breaker trips are not very frequent, there is little to wear out.
However, code has changed over the last few decades to mandate AFCI and GFCI protection. Breakers which include AFCI and/or GFCI protection have complex electronics - in many cases actual microcontrollers - and are therefore much more susceptible to wearing out, just like any other complex electronic device. In fact, many of these devices now have built-in automatic testing to help avoid surprises.
On the other hand, I have read many times that typical large appliances (refrigerator, dishwasher, clothes washer, dryer, etc.) have 10 - 15 year lifetimes. But those devices can often be kept running far longer with relatively minor repairs. In other words, YMMV.