The problem here is that Belgrade wasnt the actual capital of Serbia in 1444. It's also far too small to make an own province (we even held of on syrmia for a long time due to this concern, Belgrade would be even smaller).
It's there as the capital in later start dates though. I could maybe see an event to switch the capital and name of the smederevo province in the right circumstances but unless the map is drastically enlarged (which is not very likely at this point) I don't think a separate Belgrade province is likely.
I don't agree with you. It was considered far more important (christian defensive view) than any other right side forts on the Danube, because of the river crossings it defended, and because of the huge garrison (compared to other forts) able to be here.
Besides that I think there are two main problems here.
First one:
Map is not accurate.
Smederevo is definitely in the Branicevo province, and that way current Smederevo should be called Nándorfehérvár and given to Hungary or should be renamed something else if you really want that XVIII century borderline between provinces.
Second one:
"Historic owners and some important events of Nándorfehérvár"
circa 630-900 Avar Khagnate
900-1000 Bulgarian control
from that it was a small town with a fort mostly under Hungarian reign, first mentioned in sources as "BulgarianWhiteFort"(White Fort=Fehérvár=Beograd)
En route to the 3rd crusade Frederick of Barbarossa finds the town deserted
1284 Serbia first gets it as a marriage gift
1316 with Stjepan Dragutin's death control reverts back to Hungary
1403 In a Hungarian civil war Stjepan Lazarevic despot gets it for his help to Sigismund
1410s (till 1426) Sigismund brings Filippo Scolari to Nándorfehérvár to build a state of the art fortress.
1427 with Lazarevic's death, Sigismund in a new deal with Despot Brankovic acquires the control and it reverts back to Hungary
1440 1st unsuccesful siege by the Ottomans
1456 2nd unsuccesful siege by the Ottomans, the famous christian victory this time inhabitants and soldiers die by thousand to the aftermath: plague.
1460s Further Italian craftsmen arrive to upgrade the fortifications
1521 aug 29 The garrison finally fell to the Ottomans,
1688 sep 6 -1690 okt 8 Habsburg occupation
1717 aug 17 - 1739 sep 18 Habsburg annex, and create "crownland" on it governed by administrators coming from other parts of Habsburg realm, There territory exist under the name of Kingdom of Serbia
1739
Great Serb Migration * possible event *
1789 oct 8 - 1791 aug 4 Habsburg occupation
1806 jan 8 - 1813 oct ? Captured, held and after the Ottoman recapture suffered from revenge in the days of the First Serbian uprising
1815-17 In the second Serbian uprising, the fort itself was not taken, Ottoman garrison remained in there for another 30 years before finally leaving.
As you can see in the game's timeline Serbia held Beograd between 1717-1739 as a Habsburg puppet state, and between 1806-1813 as rebels and again this time as a "semi-indepedent" state from 1817.