I don't think the administration method is much of a distinction, poison from a poison dart frog is administered via a puncture wound, like a bite or scratch. Leathal injection is deffinetly not venom yet it is administered via a puncture wound as well. I don't know of any studies of venom ingestion, but toad venom is known to be potent when injested (ever lick a toad?).
The only difference I can see is venom is secreted via animals and poisonous via plants how ever when researching this I saw as many references to venomous plants as I did poisonous animals.
As for the origin of venom from venous, i haven't seen that in the word history (see above) "Middle English venim, from Old French, from Vulgar Latin *venmen, from Latin vennum, poison." Now the english language evolves (otherwise we wouldn't be using punctuation, and words like computer and grok wouldn't exist) and is always changing. Anyone remember when bad was good?