https://mcpedl.com/drowned-villager-mob-addon/ How do Drowned Villagers spawn?

They will rarely spawn underwater over normal Drowned, 95% chance for Drowned and 5% chance for Drowned Villagers.

They will also spawn once a Zombie Villager has drowned, just like that of a normal Zombie drowning and becoming a Drowned. Professions and Levels do not carry over, therefore they are not able to be cured once drowned.

Do they behave differently to normal Drowned

No. Drowned Villagers act the exact same as normal Drowned. It is intended this way because Zombie Villagers act the same as Zombies just with the added ability for you to be able to cure them back to normal Villagers.

They do, however, rarely drop Emeralds instead of Gold Ingots (as of v1.0.3), obviously because they are, or rather were, Villagers.

Are there any other additions or changes made with this addon?

(As of v.1.0.5) Drowned and Drowned Villagers will wear helmets correctly now. Before, the helmet texture would merge with their bloated head, and would not sit correctly on the Drowned Villager's head.