Mobil home in southern Calif.
If you mean baluster spacing…yes.
When the floor surface (landing) is above 30 inches.
Probably not required when originally built. but good call
No. In (level) guardrails it’s 4". In (slanted) handrails ballusters it’s 4 3/8". don’t get it wrong. Depends on year of construction (like Brian already said)