I look forward to see the movie.

The other thing I suppose, is that the people won't be called as Sloveni, but Slovania. The difference is, Slovania means common, undefined Slavs and Sloveni means Slovaks till the 15.-16. century (that's why Slovenky, Slovensko, slovenský, slovenčina).

Whether the Slovaks already existed as a separate nation? How can you be sure?

Speaking of nations in that time would be a nonsense. Lets say of ethnics. Slovaks had to be separated ethnic, because all of the Slavic ethnics around them identify themselves otherwise. In the north there were Polans (Poles).. were Sloveni between the Tatra mountains, river Morava and Danube Polans? No, they were not, there's no evidence they would indetify with the Polans. In the west there were Czechs (historians say of the Czechs in those times and nobody wonders, that's OK for all). Were those Sloveni Czechs? No they were not, because Czechs had complained about the Sloveni's hegemony over them and with help of Franks they fought against the Sloveni. In the east there was Kievan Rus – different state with different ethnics – not too much evidences about relations with them. In the south there were Carantanians, again another state although with similar language, but linguists are able to determine the borders between Slovak and Slovenian topographic places in the 10. cent. And the last ones South Slavs – again different states with different languages – an example : Proglas is a mix of South Slavic language from around Solun with the language of Sloveni.

So every ethnic around the Slovaks was individual with own name, but only the Slovaks are those undefined, common Slavs without any name and any characteristics. Come on ;)