These beautiful horses have an ignoble history, having been looted from Constantinople when it was sacked by the Venetians in 1204. They were later installed on the facade of the basilica, but have now been moved inside the building and replaced by replicas. (These feature in the video under bookmark 20)
Judas Iscariot was a disciple of Jesus Christ. He betrayed Christ with a kiss to the Roman authorities, who crucified him.
With the money he was paid, Judas bought a field and, in remorse, hanged himself there.
These pillars stand on the Piazzetta, near the lagoon, and were once the official gateway of Venice. They are no. 7 on the map at the back of the book (Afterword p.7).
St Theodore was the patron saint of Venice until he was ousted by St Mark (see bookmark 325). Perhaps this ousting is why Miss Garnet calls him 'spurious.' On the other hand it could be because he committed arson. Or perhaps, since he seems sometimes to have been confused with St George and with other St Theodores, she simply meant that he didn't really exist.
St Theodore is also said to have slain a dragon, to prevent it from devouring the children of a Christian woman who had been thrown to it as a sacrifice. The dragon is transmuted into a crocodile on the pillar, and this association with Egypt is perhaps on account of St Theodore's father having been Egyptian.
According to an article in the Lancet (The Art of Healing in Ancient Egypt by Rosalie David, Vol.372, Issue 9652), the Ancient Egyptians used crocodiles and fish, as well as other animals and plants to treat diseases. The efficacy of of some of these improbable cures is now being confirmed by modern scientific studies.