One of the first sentences in the story goes, 'Упади на него целый сугроб, то и тогда бы, кажется, он не нашел нужным стряхивать с себя снег…'. I'm assuming it means something like, 'even if a whole snowdrift fell on him, then it seems he still wouldn't find it necessary to shake the snow off of himself.' But my question is why is 'Упади', which I'm assuming is the imperative mood of 'упасть', in the imperative case here? Wouldn't it make more sense to have 'упасть' in the past tense using a construction with 'бы'?
In its singular form imperative form can serve as conditional mood, so the sentence "Упади на него целый сугроб, то и тогда бы, кажется, он не нашел нужным стряхивать с себя снег" can be rephrased as "Если б на него упал целый сугроб, он и тогда бы …".
Here are some other examples to give you idea:
- Знай [бы] я тогда, чем дело обернётся, ни за что бы в это не ввязался.
- Поверни [б] я чуть раньше, не стояли б мы сейчас в пробке.
- Поверь я ему тогда, сейчас, возможно, меня бы не было уже в живых.
It is a variant of Russian conditionals that is considered dated by some linguists and not commonly included in grammar textbooks, yet frequently encountered in educated people's speech. The imperative, instead of the subjunctive, is used to indicate a very strong belief, one held as a law of physics — or, in this case, to liken the implication to such law for an ironic effect.
The closest interpretation in English will look like: "If a whole snowdrift falls on him, then he still doesn't find it necessary to shake the snow off of himself." Note the weird use of zero conditional — if the story was written in English, zero conditional would have found use to make fun of the character's excessively rigid habits.
I want to emphasize that I do not know the scientific explanation, but I want to express my opinion.
You have a correct understanding of the meaning.
"Если бы на него упал целый сугроб..." sounds with a connotation of the past. "If you had told me before, I would have helped you" — everything is in the past.
"Упади на него целый сугроб..." sounds with a connotation of the future, with a connotation of hypothesis. If this could happen in the future and not in the past. "If he could have a million dollars, he would save money even then". The imperative gives a connotation of the future. "Clean your room". Means do it in the future, a person cannot clean it in the past.
However, the direct meaning is the same.
I found information that the imperative's use as a subjunctive mood is very rare and not normative.
I think the author did it for a purpose of artistic expression.