can 'in case of + noun' be translated as si + genitive, e.g. 'si ignis' (in case of fire)? or is a verbal clause (i.e. si forte + subjunctive) more idiomatic? thanks!
No, a sī-clause cannot be nominal, it needs a predicate; in absence of one it reads just like 'if a fire' in English, i.e. as a elliptic list item with the same verb as before understood. The only possible verbless use is as in sī quisquam, ille 'if anyone then surely him'.
'in case of X' has two major uses:
as a provision for a likely event, you basically want to translate 'if there's fire', which is easy: sī incendium fuerit; likewise, 'in case a tree is felled by lightning = if a tree falls after being hit by lightning (call your local spirit guide for further instructions)' sī arbōs fulgure icta ceciderit. The perfect tense expresses the sense of definiteness & punctuality which in English distinguishes 'in case of' from a simple 'if'; the latter expressed in Latin as sī incendium erit, sī arbōs cadat.
- notice that ignis refers to the light-emitting effect of combustion, and not to an area or property that's on fire.
A couple of ready-translated examples from Livy's Loeb, indicating ongoing states (the narration is present but the events are in the past):
Senātus magistrātibus in forō praestō est, sī quid cōnsulere velint [...] aliī offerunt sē, sī quō ūsus operae sit. ('The senate awaited the magistrates in the Forum, in case they wished its advice about anything [...] others volunteered, in case of any need of their services.'
as a casual precaution for an unlikely event ('you never know'), that's when you need forte 'perchance': sī forte incendium erit, aliā perveniēmus 'in case there's a fire (in our way), we'll get there by another route'; Lūcium sī forte vidēbis salvēre jubētō 'in case you happen to see Bob, pass him my greetings'.
When listing alternatives, these are introduced with sīve~seu 'else if': sī A, B; sīve C, D...
With negation, sī becomes nī 'if not' and sīve becomes nīve~neu 'and if not' (often spelled NEI and NEIVE). This one smacks of legalese and archaic language, to be used on occasion:
id nī fit, mēcum pignus sī quis volt datō ('if this isn't the case, let anyone who wants bet against me', Plautus)