Yes, I don’t know how to solve either 
But two minds better than one.
I’m looking at the mbed-mcuboot-demo on GitHub. The mbed_app.json file has more overrides listed than shown in your mbed_app.json file. Are they not required… e.g. scratch-address and scratch-size is needed for secondary boot sector.
"NRF52840_DK": {
"target.features_remove": ["CRYPTOCELL310"],
"target.macros_remove": ["MBEDTLS_CONFIG_HW_SUPPORT"],
"mcuboot.primary-slot-address": "0x20000",
"mcuboot.slot-size": "0xC0000",
"mcuboot.scratch-address": "0xE0000",
"mcuboot.scratch-size": "0x20000",
"mcuboot.max-img-sectors": "0x180",
"mcuboot.read-granularity": 4,
"qspif.QSPI_MIN_PROG_SIZE": 4
},