I don't believe hard determinism can be reconciled with Epicureanism, but to be fair lets say it can. You would need to tackle the swerve, because as long as the swerve exists a cause or set of causes can lead to a number of effects or none at all, thus determinism cannot exist. Without the swerve you effectively render epicurean physics Democritean.
The Epicurean swerve is an event in an otherwise causal universe (and one in which, as you pointed out, there can be multiple causes for any identifiable effect, and multiple effects from any identifiable cause). Therefore, it seems to me, it does not entail a universe of total indeterminism either.