Why does capital flight cause inflation?
(1) Shouldn't it cause a deflation? Currency is less and less
  available (due to flight) for the purchase of domestic goods, thus..
  deflation. That is, a smaller amount of currency chasing the same
  goods.


The dollar is less available, whereas the purchase power of the bolivar (Venezuela's domestic currency, apropos of your example) becomes negligible: To buy the same goods, today Venezuelans need more bolivars than in the past. Inflation comprises two factors: economic activity and purchase power. In cases like this, inflation is driven up by the bolivar's drastic loss of purchase power.


  (2) But also, capital flight is done with saved money, instead of
  money reserved for purchases, and that means aggregate demand is not
  affected much, meaning, purchases are done as normal, and normal life
  follows. Surely, that doesn't mean a deflation, but it doesn't mean an
  inflation either.


Capital flight results in a reduction of supply. If demand remains unchanged, then the intersection of supply and demand curves occurs at a higher equilibrium price. That means that the bolivar has lost purchase power.


  (3) Consider this: Assume for whatever reason, interest rates has
  fallen. This means, borrowing money is cheaper, and that is likely to
  cause a shift in aggregate demand and domestic consumption, possibly
  boosting the economy, and causing some inflation in the process.
  However, with capital flight, there's less and less investment done
  inside the nation, so, the supply of investment shifts, causing an
  increase of interest rates -- which again, should tend to cause a
  deflation due to a decrease in the money supply (basically, the
  reverse reasoning of lower interest rates).


It is difficult to make conjectures where a situation gets out of control. In broad terms, the relation between interest rates and money supply can certainly break down when chaos strikes the markets. But even in a less-than-extreme scenario, that relation may hold and yet the net effect can still be a higher level of prices.

Also keep in mind that printing extra money to honor the government's bonds contributes to depreciation of the domestic currency, and interest rates might not be high enough to attract lenders.