Royibernthal,
It is a shame Unity doesn't support C++. Considering it's likely to survive in game engines for 10 or over years.
A conversation about that if you're interested: http://gamedev.stackexchange.com/questions/37361/will-c-remain-viable-for-game-engines-in-somewhat-distant-future
I'm going to answer each of those questions directly for you!
1. Career-wise, would it be a wise choice to learn Unreal Engine 4 / CryEngine 3?
Yes, it would be a good career option and out of the two of them I would definitely suggest going for Unreal Engine 4. It's more widely used by games dev companies then CryEngine 3.
2. Would they open doors for me in the future, not only in the companies behind those engines?
Yes. I think learning to code in any engine will open doors for you in multiple companies, especially with the massively expanding market. This especially stands out for Unreal Engine 4 - although the engine isn't publicly available yet it will be around and possibly the game engine market dominator for many years to come.
3. Would any knowledge I gain from working on these engines be useful in other engines I might encounter in my career?
I think a good understanding of any 3D game engine gives you potential to work on other engines since most engines share the same attributes. I used to work with a Unity developer who had never before used Unreal Engine but managed to conquer it in a matter of days because he had a basic understanding of how the engine worked.
4. Would the experience add to my resume?
Absolutely. If you're looking to get a job in the games design industry that is to do with modelling or coding - you generally need a understanding of game engine coding/general development. Teaching yourself to code in say, UE4 would be impressive on a resume.
5. Alternatively, are there any other engines I should consider? Not because they are easier to get into, but because they seem like a good choice.
Good question. With the ever-expanding market in the games industry, more and more engines are becoming publicly available. You mentioned CDProjektRed in your post earlier (brilliant company by the way, love those guys) they are in development of their third witcher game which is running a new game engine, rumoured to be as powerful as Frostbite 2.
You'll find most big companies that know exactly what they want in a game will build their own engine for it. You'll also find that they will train you in how to build in that engine which is why you'll need experience in a well known semi-complicated engine, such as Unreal Engine 4.
Have you ever heard of a game called Infinity Universe? Those guys are building their own engine which generates and hosts thousands of planets automatically, they've been in development for a long while with barely any funding (that I know of, don't take my word on that) and they're still churning out some amazing looking stuff. You can check them out here http://www.infinity-universe.com/Infinity/index.php?option=com_content&task=view&id=12&Itemid=33
As Mathimetric mentioned, OpenGL Library is an option. Find that here: http://www.opengl.org/resources/libraries/
I hope this answers more of your questions.