I assume you're still in the "inspection period" in which you can back out of the sale and get all of your earnest money back.....if you want to do that
I don't think ??? things that aren't up to current code are necessarily a deal-breaker......if the building was built to code at construction and the code changed since, those things would be grandfathered in, I think......much like old cars are still allowed to drive on the road today, despite the fact they break all pollution and other safety codes today.....
sounds like the retaining wall is definitely a negotiable item - mostly because of the collapse threat, not because it wasn't built to code (it probably was built to code at construction and the earth has shifted since????)......you don't have to "completely re-negotiate", but only need to negotiate this one item as a response/request to your inspection......your realtor should help you with this.......
the outdoor stairs difference in height thing and similar stuff is not a real threat or defect, in my mind, but can be a negotiation tool......personal preferences and taste shouldn't be used as a negotiation tool, of course, though......
if you have your heart set on this place, you may be willing to forgive some stuff......if buyers are lined up, though, the seller may reject all your requests for fix-up and move on to the next hot buyer (unless you love the place and decide to fix everything yourself)