Yesterday's MR fixed the search engine problems, and they will skip this schema, more or less, but I decided to investigate a little bit more.
The initial implementation was in 179d2c89, then the file (toolkit/components/search/schema/search-engine-config-v2-schema.json) has been renamed in 5aacd4b8 to toolkit/components/search/schema/search-config-v2-schema.json.
In general, there are a few other schemas in toolkit/components/search/schema, but they don't seem to be too complex.
The JSON I added to the MR doesn't pass search-config-v2-schema.json, but I'm for a YOLO approach to it (it works, it's enough for me).
So, this Bug itself was not that worrying/we bypass it if we use this approach.
However, I checked also the related Bugs, and found these ones:
Bug 1818323: "Search engine configuration parity with Desktop": it seems the goal was to apply the same config also to Android (!)
The most interesting (public) bug seems to be Bug 1885953, which is about deleting former web extensions (not solved yet).
I'm following it on Bugzilla, I'll follow up also with a new issue downstream.