When do you pronounce Roman numerals as numbers and letters?

Roman numerals represent numbers, just as the more common Arabic numerals (0,1,2,3,and so on) represent numbers.

So, you would pronounce the individual components of Roman numerals only in cases where you would likewise pronounce the individual components of Arabic numerals.

No. Roman numerals are never pronounced phonetically. Also, when they are used as a suffix to a name or place, they are always read as John Doe "the" Second (or whatever other number). Note that there are also very few examples of the use of roman numerals in common vernacular, and they are used almost exclusively in formality.

"second" and "thirty" are both words for numbers. What you mean, I think, is that sometimes Roman numerals are used to indicate ordinal numbers like "2nd" and sometimes cardinal numbers, like 30. You can read them as ex eye ex, but it's going to be harder for people to understand you, as they have to translate them in their head.

Yes. I can think of two cases.

(i) The rocket ship 'Fireball XL5'

https://youtu.be/6ifS2nP53Zs

(i) The German 'V1' doodlebug rockets in WW2,