They exist for sure if you're a tourist. You're either a payday for a local cop or an example to be trotted out as proof they enforce those laws. The latter is so a higher up can divert DEA funding into their bank account.
Obviously that's not every developing country, maybe not even most. But unless you're very familiar with the country and area, it's not a risk you should take.
Exactly. Whether you're in the kind-of or kind-of-not has everything to do with bribes and/or local connections. And how many other people are also breaking that particular law, like in Jamaica's case. Sometimes this is bad, sometimes this is paradoxically helpful. Technically homeless people in the third world can often build a permanent house anyway, for example.
It also varies significantly by level of urbanness, with cities being the most formally run. Even backwoods first world will have things going on that would never fly in the cities.
They recently decimalized it and legalized medical use. Getting a medical card involves filling out a form self proclaiming a need and paying a $15 filing fee. You can do this in the dispensary while they are bagging up your weed and psychedelic mushrooms.