1) Unfortunately, surviving records are too fragmentary and incomplete for the entire lineage of the Princes of Gothia to be reconstructed with certainty, let alone accurately dated.
2) Alexander replaced Issac in a palace coup just before the principality fell, in 1475. This is recounted by the renowned Byzantine historian Alexander Vasiliev:
In 1475 Isaicus, whose friendly policy towards the Turks probably aroused discontent among his subjects, was overthrown by his own brother Alexander . . . An Italian vessel took Alexander on board in Moncastro and brought him to Gothia. Evidently Stephen the Great played a very important role in Alexander's undertaking, because the Hungarian ambassadors at his court in their report to their king, Matthew Corvinus, dated June, 1475 state that 'some time before that the Voevode Stephen had sent Alexander, his wife's own brother, to the Principality which is called Mango, i.e. Mankup . . .
Alexander was very successful in his expedition. On the third day
after he had landed in the Crimea, he overthrew and killed his own
brother Isaicus and took possession of Mankup, his "paternal heritage."
Vasilʹev, Aleksandr Aleksandrovich. The Goths in the Crimea. Mediaeval academy of America, 1936.
3) Demetrius is the first Prince of Theodoro to be known by name. There is no historical record on when or how the principality became independent from the Empire of Trebizond, nor indeed of when the principality was established. Nonetheless, due to Trebizond's remoteness and relative weakness, Gothia's dependence was never very strong in the first place, being largely confined to an annual tribute. It is possible that the Princes of Gothia merely substituted nominal subservience to Trebizond for Tartar overlordship.
If we look further back in history, prior to Trapezuntine suzerainty, the Gothic Climata were part of the Byzantine Empire. Imperial power was constantly in flux over the centuries, however. It has been argued by Vasiliev that Crimea in fact broke with Constantinople shortly before the crusaders sacked the capital in 1204, as it is omitted both from the imperial treaty with Venice in 1198 as well as from Latin partition of the Byzantine Empire in 1204. Within a couple of decades the former Byzantine possessions in Crimea fell under the control of the newly established Byzantine rump state in Trebizond, though.