I think the stuff about bringing it "up to code" is nonsense, in this
case. This isn't a new house, this isn't new construction, addition,
etc.. I'd be surprised if there even was a "code" to speak of in this
case, which means that whatever your building inspector says is okay,
is okay, and anything else isn't. If you involve your building
inspector, that is.
If this has been 30+ years, with little to no movement in that earth, I
think starting in on a massive excavation and foundation job would not
be called for.
First, do you know how thick the wall is at that point? It could be a
simple 8" thick wall that ends abruptly. Or it could have a thickened
half-footer edge (i.e., a footer but only on the outside of the wall,
so not visible from inside).
Your idea of just adding some concrete at the corner of the wall and
floor is done pretty often around here, as far as I can tell. We had an
offer to do our garage basement this way (but we didn't end up doing
that). It would be much cheaper than any alternative, and likely work
just fine -- get some more opinions from foundation and concrete
workers. It involves first drilling into the floor and bottom of the
wall, and inserting rebar pegs. Then you build a small set of forms,
and pour a beam (maybe 1 foot high by 1 foot thick) around the edge.
The rebar pegs help bond it to the wall and floor. The purpose is to
take some load off the soil under the wall (via the rebar pegs,
transfering load to the 1 foot wide new concrete base), and to keep
that soil from moving. With a concrete pump, a team of workers could
have this done in a few days.