There are four Great Plague churches in Venice, but according to the author, this one is fictitious.The fictional Chapel-of-the-plague is marked '3' on the map at the back of the book (Afterword p. 7).
The Hotel Gritti overlooks the Grand Canal.
You can see some of the grandeur here
A more prosaic explanation for the camel is that the brothers made their money trading in spices from the East.
Candlemas (2nd February) is a Christian festival celebrating the presentation of the Christ child in the temple, and the purification of his mother after childbirth.
It became known as Candlemas because candles were traditionally blessed on that day, for use throughout the year.
The king Tobit served would have been Sargon II. Shortly before he was killed in battle in 704BCE, Sargon had a new town and palace built twenty kilometres north of Nineveh. This was presumably the palace where Tobit lived within the walls and walked in the gardens.
When Sennacherib succeeded his father in 704/5 BCE, there was an uprising in Judah, which must have put him out of temper with the Jewish people. Sennacherib was noted for his architectural achievements in Nineveh, where he built another new and magnificent palace.
He was murdered by two of his sons in 681BCE (see p.74).
In the Bible story, King David's son Absalom had his brother Amnon murdered in revenge for the rape by Amnon of their sister Tamar. He spent several years in exile, but father and son were eventually reconciled. (2 Samuel 13 and 14.)