For instance: is it feasible for a country to be at a 1890-1912 tech level, whilst the rest of the world is more near 1750-1820?
In a world of humans who undertake rational economic behaviours, I would say no, at least in the long-run. For this question, I will also be interpreting 'tech' on the lines of productivity, economic production, and military technologies.
Any technologically advanced nation will require large amounts of economic output to support that technology, that society, and the consumption which made that technology possible. In a world where there are lots of people everywhere, i.e. the technologically advanced nation does not have a monopoly on the world population, there are two major mechanisms, which I feel, would be rational to pursue, and which would lead to the diffusion of technology.
Colonialism and imperialism
The first would certainly fit into a 1890-1912 period. Colonialism and imperialism are results not out of evil persons and character, but rather, the technological, economic, and military domination of a certain society over another. I would argue that something like European imperialism was not caused by Europeans being horrible people, but rather, by the fact that Europeans had a massive technological advantage over their neighbours. Colonisation would lead to the diffusion of productive technologies as the primary purpose of empire is economic and political. Empires cost money to maintain, thus, the metropole will want to extract economic value out of it. Or, they will do so due to political concerns (e.g. British invasion of the Sudan in the 1890s to check French expansion into the same area).
This means that railroads and other pieces of infrastructure will be built. This will necessarily diffuse technology, because railroads have to be maintained. Military or political networks require logistics. All of these will require employ of the native population, which will require that they assimilate technological advances from the metropole.
Furthermore, as colonialism and imperialism will also be resisted by indigenous populations, those populations will also likely assimilate technologies necessary to strengthen their societies, expanding their economic production and war-making powers. This means that such technologies would also diffuse to nations willing to reform.
The second major mechanism would be foreign direct investment. Today, the surplus of labour in developing countries and low productivity means that wages are low. This gives strong incentives for corporations or organisations from the metropole to invest in foreign nations, and therefore, spread production technologies, which would allow for massive expansions in economic production, as we see in places like China and south-east Asia today.
Even if there is no foreign direct investment, entrepreneurs from one nation will almost certainly steal or copy technologies from abroad. The production of silk was heavily regulated in China. It travelled to India anyway. The technology of water and steam-powered looms was heavily regulated in Britain. It travelled to the United States anyway. This is simply because a market opportunity exists, and it will be filled in time.
The real-world examples of massive technological gaps, like Britain and Afghanistan circa 1912, have to do with the fact that places like Afghanistan are effectively unconquerable and unable to support Industrial age technologies (for various reasons). It would probably break the wilful suspension of disbelief for the rest of the world to basically be such a place.