Yes, any MS user is capable of it.
Only full blooded Uchiha's can do it.
No, only certain MS users can.
And i still stand by my statement, only Madara was and still is capable of controlling a Bijuu.
But the kyubi was inside naruto all along. How was itachi supposed to control it if naruto was already though so?
Madara had an excuse, he had an angry army ready to recieve a royal beatdown, he was too pre occupied, and even then he still found the time to try and summon his loyal beast.
I don't think it is necessarily extremely easy and we don't even know all the details we would need to know on the matter. For one thing, when sasuke sent the kyubi back in naruto's mind it was not naruto who was in control, it was the kyubi. The sharingan control's biju and biju control their own chakra. If sasuke takes a stroll in naruto's mind and takes some form of limited control over it (which as the databook pointed out is possible given the manda bit) it is viable to suppress it. On the other hand, nowadays it is not the kyubi who is in control but rather naruto. The ability to control biju should not be that useful if the one controlling the power is not a biju for one thing. Then again, if to control the kyubi itachi would have needed to take a stroll in naruto's mind then the situation would not have been so easy for itachi. Naruto is immensely more powerful than what he was when he fought sasuke all those chapters back. Of course, itachi could put a genjutsu on naruto however the kyubi would release him before the stroll for control is viable. Even if all those things were not an issue, itachi and nagato were fighting not 1 fully powered jinchuiriki but 2.
I still think it is a matter of talent and skill. Anyone with the sharingan could IMO be in possession of the technique which controls biju, it is an innate ability of it. The difference would lie in skill, not necessarily anyone with the sharingan would have the skill or talent to pull the technique off.
Another thing I have been thinking is why exactly was tobi able to control the kyubi though. He certainly did not have a MS so it is kinda against what the manga has shown. This made me think on what the databook said about sasuke controlling manda which suggest that even a normal sharingan can to some extent pull of the technique even if only at limited fashion. So far we have strong evidence to suggest tobi's body is actually made out of zetsu or something of the sort. What if having a body made out of hashirama's cells strengthened his sharingan so as to be able to do whatever the normal sharingan can do except just a bit better and essentially rid himself of the need of a MS?