As far as I know bubur is a congee (or rice porridge), since most Westerners (well most Englishman at the least) use porridge to mean oatmeal, I think the fact that it is a rice porridge needs to be emphasised.
"congee" is a good alternative translation (imho).
However, please note that "bubur" does not necessarily mean rice porridge, although that's probably the most popular type (bubur ayam).
For example: "bubur kacang hijau" (one of my favourites) is not made of rice.
Here are some recipes:
Not when talking with Indonesians, they would always assume porridge means bubur. I understand your point ... that we need that clarification. Indonesians would ask in English, 'Would you like some bubur?' They wouldn't translate bubur at all.