1. It would probably be a bit odd to give Constantinople a level 2 fort, while a mere level 1 fort is underwhelming. Simply having the province start with a defensiveness bonus I think would work out well; something akin to the 25% defensiveness in a province event that can randomly fire. The tricky part here is to give them something that makes Constantinople the most defensive position in the world at game start, but doesn't last all game and also doesn't become unbalanced when it's AI vs AI and the city is Ottoman by 1460 almost every game. 25% and just the regular ol level 1 fort seems good to me.
2. Byzantine ideas could be better. A galley combat bonus in traditions would probably make everyone happy - even just a small bonus here will do HUGE things for most strats which involve sniping a subset of the Otto's fleet and blocking the straits. In fact, if we're going to throw a buff their way, Galley CA in traditions is probably the best way to do it (over more cores etc).
3. Eastern Tech is fine and appropriate; the problem is that eastern Tech is magically worse than western, not that Byzantines are in that tech group. It allows them to field Eastern units and cavalry ratios, which is totally necessary for the Roman rump state renown for a focus on heavy cavalry. The only issue is obviously the tech malus. Take it up with the devs for designed tech groups with flat and arbitrary penalties - if you think Byz having a tech malus in the 15th century is the most absurd, Ming would like a word with you.
That's not really the point, the point is players should be rewarded when they successfully pull of playing difficult nations, and difficult to play nations should also have their perks. Especially ones that make sense and are historical without being too impactful allowing players to forge their own nations after the initial historical setup of 1444.
What kind of rewards are you looking for? Perhaps a triumph decision for retaking different regions? Free monarch points when you reclaim major cities like Antioch? Hell, let's go all the way, how about we beef up the development of the capital if you're able to reconquer the large core region of Greece and Anatolia, maybe call it the theme system?? The Byzantines have more developer attention and focus than all but THE most fleshed out nations. Very few compare. There isn't a single nation in the game with more events and love than them, that can't easily be justified as a major and influential presence throughout all or most of the period.
Yes because it takes 200 years for people to flee from Greece/Turkey/Balklands to the rest of europe. For them to start causing the Renaissance 200 years later. The fourth crusade did cause refugees to flee to Europe, however it was the fall of Constantinople that revived peoples interests in the old manuscripts brought by the previous refugees (now reinvigorated interest from the new refugees with newer modern manuscripts after the old manuscripts fell into obscurity), this kick started the process which caused the Renaissance in the mid to late 15th century.
The problem here is this notion that only a more advanced people will contribute. Unlike the game and unlike maybe more modern sciences, "tech" at that time was a lot less linear in real life. An influx of now-landless foreign professionals into your society IS going to help spread advances, even if your own society was relatively ahead on the whole. Sorry to Godwin here... I picture it akin to the German scientists which fled to the USA after WWII. Sure, the USA's tech was great, but getting the cream of the scientific crop from a foreign nation still helps you advance further.