With regards to the cores issue:
AT start (1444) I would only give Byzantines cores on Greece, Macedonia, and Thrace (Morea, Achaia, Athens, Naxos, Epirus, Thrace, Edirne, Albania, Macedonia, Kozani, Larissa, and Rhodes provinces in DW) If Byzantium secured these areas they could enact decisions to give them cores on venetian possessions, Bulgaria, and the Anatolian Coast, and Serbia.
With regards to the religion issue:
Byzantine submission to Catholicism was mainly just lip-service, but it did cause unrest nonetheless. Byzantium should start with a modifier that gives additional revolt risk but boosts relations with all catholic countries, and can be removed via decision at the cost of killing relations with said catholic countries.
With regards to the straits issue:
While it should be remembered that the Ottomans never really built a significant navy until they set out to conquer Constantinople, a hallf-dozen Byzantine galleys should not be able to block both the Dardanelles and the Bosphorous. I'm in favor of the heavy attrition suggestion, with a fleet only able to block a strait completely if the number of ships is equal to the number of regiments.
With regards to Orban:
for those who don't know Orban was the Hungarian gunsmith who designed and built the ottoman heavy artillery for the siege of Constantinople. However, he initially offered his services to the Byzantines. There should be an event in-game where Orban offers his services to the Byzantines in 1452 and for 100 gold they can get research points invested into the Land Technology category. For 160 gold the Byzantines should also be able to upgrade the Fort Level in Constantinople. If the Byzantines decline then it should trigger an event for the ottomans where they get the same options.