SolarDIY

I wouldn't be that surprised if it was fine. Much of the time inline devices like this are designed to sink heat into the cables, and I've definitely seen some fuses that get pretty warm from being run more than 50% of their rating. They kind of have to in order to work; it's not like the fuse can remain perfectly cool at 300,350,399A and then immediately melt at 400.

I can't say what is what here from specific experience. Use some fine steel wool on the contact surfaces of every connection to polish it. Be VERY careful with steel wool around batteries as it will catch fire instantly if it bridges a large voltage potential, like from dropping it.

When I am working with circuits and etching, steel wool is best for removing every bit of the oxide layer, ensuring a much better connection/consistent surface to work with.

You generally need a 5+ digit digital multimeter to get into the range of measuring shunts directly. An ancient old shit brown ugly keithley 197 will generally work and can usually be found second hand for around $100. Then you can directly measure shunts with a 4 wire probe (current source and sense wires are separate to eliminate the wire resistance from the measurement).