Yup that's possible but only to a certain level.
No, it can be done up to the highest difficulty levels. If you want an example, you can check my active story (deity level + bad start + overcrowded = furious tech pace in the ancient ages) where i got Literature first, and with minimum research spending. Besides that, people have snatched monopoly techs even at Sid level.
The trick is to have a general idea about what the AS are likely to research, and go to the route you think they will neglect. In the AA, you get the best chances with Literature, but you can also succeed with Writing or Philosophy. In the MA, after the 1st tier techs, the AS usually go to the lower tech branch, so you have good chances with Theology and Education.
Mind though, it does not always work, there is some luck involved. If, say, the AS gets a lucky pick on a barb hut, your plan may be screwed up in an instant.
At late medieval you better have an empire outmatching everyones or you will be fighting with tanks when they have modern armor at best.
In most cases, my games end in early to middle medievals, but if the game drags itself up to that point, i do, usually without a single university
Sure, you can play very smart with tech-embargoes on several nations that you then switch and start trading tech to them while hoarding gold to then boost a new wave of research but at some point it just starts getting gamey and tiering at least for me.
Trade embargoes are just not worth it. You get better results by trading as much as possible, both techs and luxuries, and keeping your reputation clean. In the middle ages the AS tend to lose their edge quite dramatically, so you need to keep them healthy in order to be of any help. That is, unless you choose to go berserk and destroy them one by one
The reason why you are having a hard time in the tech race may be some basic mistake in the developing phase, such as not training enough workers / not improving the land properly. Land improvement is paramount in every scenario. No roads means no money, and no money means no research.