When an actor-focus verb has both mag and um forms, are both options equally valid for all contexts?
In general, yes, because "buma
sa" and "magbasá
" both mean "to read".
... what factors may affect the choice?
Regional/local usage and/or personal choice.
The response to your saying “nagbasa ako ng libro” in Bataan was based on their regional/local usage there.
I was born and raised in Manila as my parents were, too. “Nagbasa ako ng libro” is how we say it here. If I hear someone use “bumasa” instead of “nagbasa” in that sentence, I would think that the person is not a native of Manila or he learned Filipino from someone who hails from another Tagalog or a non-Tagalog region.
Although I can consider both words interchangeable, I would use “magbasa” to refer to reading things with words or sentences on them, which we encounter about 99% of the time.
On the other hand, I would be inclined to use “bumasa” to mean rudimentary reading, as when a child is just learning to read the letters of the alphabet or about “reading” abstract or conceptual things, e.g., the reading of numeric symbols, tarot cards, and signs/symbols like 🚸💲🚷💈♻️🕘🇺🇸🛑👍🎵... etc.